Long post incoming:
Premise:
The Night Warrior story is limp for many reasons, but two of the main ones are these - we're told constantly Tyrande is more powerful than ever and also more aggressive and hostile towards the Horde than ever, but neither of these are the case. She does nothing impressive throughout her stint and the sum of her more warlike attitude is fighting people who were fighting her anyway and sulking at Anduin then going after Sylvanas exclusively. A motive so compelling and unique that everyone else in the paint drying seminar that is the heroic cast has the exact same one. On a basic level the story fails.
But what makes it even worse and harder to correct is the situation it resides in. BFA is not very good, and the overall message of forgiveness and universal love is bonkers, for none more so than the night elves. The main message of the narrative is thus off, but to add to it the specifics of the narrative fail also. Tyrande has next to no screentime in BFA after 8.1, which leaves her with one major appearance and some minor conversations, book appearances and then Shadowlands where she'll lose the power up. This screentime is mired in terrible plot points, not the least of which everything relating to Darkshore - there's no emotional stake in a Forsaken vs. Night Elf story as the two have had two NPCs worth of interaction before BFA and even in BFA it's comically lopsided how much more invested the night elves are in the fight. More so than that, the undead night elves operate in a weird limbo where they're key to several moments with Tyrande but are abysmally motivated and serve no large scale purpose one can follow. Would the story be exponentially better if it was orcs instead of or alongside Forsaken and if the undead night elves didn't exist? Yes, unquestionably, much like the story'd be greatly improved if the war wasn't bound to end as absurdly as it did. But if you ditch all of these things and consequently also let her keep the mindset change and power-up as it's more interesting then you're not really telling remotely the same story. So for the purposes of this post, we have the following restrictions - we have to keep the same time frame and same number of appearances. We have to keep the Forsaken in Darkshore, the undead night elves and the general plot of BFA can't change. Tyrande must become the Night Warrior in 8.1 and the nature of the power-up has to change at 9.1, the same people must be alive and dead by the conclusion.
The Night Warrior as a Power-Up:
First off, we change how the power-up works to better emphasize how important it is. In-game, we're told that the Night Warrior is very dangerous and powerful and makes you angry, not why. In this version we'd borrow from the only prior lore of the Night Warrior, which is that she collects the souls of fallen warriors and sends them with her, like an elf Wild Hunt. This'd be the basis for the Night Warrior's power - the collective strength of the souls she'd be taking into her care and with her emotional state being the result of being constantly under pressure by their wish for vengeance. This makes her an obvious problem for Sylvanas's underlying plan - the Night Warrior skips the soul-selecting process, souls don't go to Oribos so they don't go to the Maw and gives her a unique role that no other figure has in circumventing the Jailer. It also helps sell how unnatural the power is in that it disrupts the way spirits usually flow and helps make the comparisons to Sylvanas less tortured - now both hold their powers on the basis of taking souls away from their destination, with Tyrande doing it for good and to save her people on what she believes at the start is a temporary basis. It is also self-balancing - she can save all future night elf souls from going to hell, but not the ones that have already gone so the more she saves the more powerful she becomes. An even bigger benefit of it though is that it means by simply existing and being alive while night elves die and becoming the shepherd of their souls, Night Warrior Tyrande is accomplishing something and foiling the baddie's plan to some extent.
Patch 8.1:
Moving on from the premise to the actual events involved and since we have to keep the undead night elves around, the timing would be hte same change. From the Horde perspective, all but one of the quests would explicitly take place prior to the Night Warrior ritual, including raising both Delaryn and Sira, who'd be ambushed - she'd have been ordered to hold back by Shandris, but seeing Delaryn being raised, she'd join in and be killed and raised herself. Tyrande, who'd be off doing the ritual, would not be there to help her rather than five minutes away, hence her abandonment issues. Both Delaryn and Sira would be proofs of concept that the Val'kyr can now raise night elves, something that'd surprise Nathanos himself. He'd give a quest on the beach mirroring the Darnassus one of raising as many as possible, but no sooner would the PC arrive there and Brynja start raising them but would the moon turn and the Val'kyr'd get smited, losing most of her HP. Night Warrior Tyrande'd emerge and engage the PC, Nathanos and some troops. Instead of two Val'kyr, there'd be three and Skyja would be the one chaining Tyrande, Brynja preparing necromancy and Signe focusing on Nathanos. At this point, you'd do the cut you do before, except instead of Malfurion showing up, it'd be Tyrande showing off what the Night Warrior's power entails by countering Brynja's raising and the phantom warriors overwhelming the group - she kills Nathanos and the Val'kyr fly off while Tyrande saves the souls of everyone on the beach by making them part of her army. At camp, Brynja dies to raise Nathanos on Sylvanas's orders, Skyja commenting on how that's a waste and the warfront would go on as normal.
Alliance quests changes follow from there. No moment of failure or necromancy, you help Tyrande do the above, and you up the awe factor of Tyrande and her phantom forces - ghostly troops are a prominent feature of the Alliance army Darkshore-wise and it's emphasized both how necromancy is now useless in her proximity and how the wildlife as a whole is going mad in proximity. Additionally, Tyrande carries out the ritual herself - other Night Elves
don't get the eyes. That's for later. In changes on the Warfront itself the skybox is altered to get across that the Horde quests occur first - with Sira as the commander and the moon being normal, this being the Horde takeover, and the Alliance version being post-Black Moon and ending with an explicit liberation of Darkshore by Tyrande's forces. The final bit of the Warfront would be a setpiece where she and her forces help you team up against a stock Forsaken general or maybe vs. Brynja or one of the other val'kyr instead of Sira, who'd escape, because she has contractual immunity.
Away from Darkshore and when it comes to the main conflict, Tyrande explicitly refuses to send any troops who don't explicitly volunteer to Dazar'alor even pre-Night Warrior. Shandris is the one to agree to help, not because of her love of human potential but to take some heat off of her mother and ensure she has a shot at Darkshore and the night elf forces in the suicide army are under her explicit command and don't march single file to their deaths but get a token quest where they do some guerilla sabotage. None have night warrior eyes or use Darkshore assets, after the raid, Shandris mentions she'll stick around to take care of her people who'll want to help the main Alliance effort and to serve as her mother's voice in Anduin's circle as the Night Warrior won't do it herself.
Patches 8.1.5 -> 8.2.5:
Because the narrative gargles gonads and we're bound to follow its rhythm, we don't actually get back to Tyrande explicitly until 8.3 and even then she's stuck with a cameo. Still, there's lines to emphasize here that'll kick in later when we get to Shadows Rising. 8.1.5 would have throw-away lines relating to how the Alliance's turnaround is chiefly on the Kalimdor front and while Darkshore has been reclaimed, Tyrande is now moving into Ashenvale, with her going ahead of most of the forces. Shandris is the one to tell this to Anduin. Her appearance in the story also change - she's not originally in Nazjatar but heads on in after the fleet is sunk. She mentions in flavor dialogue that Tyrande would not budge from her fight with the Horde even for Azshara, which surprised her. Horde-side Nathanos' throw-away dialogue in 8.1.5 also brings up the push-back in Ashenvale and other NPCs react negatively to the news - to keep her core constituency on her side, Sylvanas is committing heavily to that front. This is what ultimately allows the Alliance and the rebels their opening to go after Orgrimmar directly as even after the fleet is destroyed, Sylvanas isn't free to act. Night Elves don't however come to join up with Anduin under the walls of Orgrimmar, hence why he's understaffed, the reason for this comes through in 8.3.
Patch 8.3:
After a lot of hearing about Tyrande we get to see her again in this. The Alliance leadership meeting starts with Shandris speaking for the Night Elves, not expecting Tyrande to appear at all. Shandris is not positive about the ceasefire but recognizes her people's overall exhaustion. Tyrande's appearance is a surprised, bringing the black moon over Stormwind when she does. The confrontation and break with Anduin is more vicious - he freed Saurfang, with the spirits she's thus far assembled calling for his blood and for that of the Horde. She all but withdraws from the Alliance on the spot, with only Shandris talking her down to the point where Tyrande says that this ceasefire won't be acknowledged and he can take his chances with the Horde if he wants to. She means to finish the job - Sylvanas first, then the rest, reminding him of how ever since they joined the Alliance they've been the first group targeted by the orcs and taken the brunt of the damage while the Alliance reaped the benefits and that a simple change of leadership means nothing. Told of N'zoth, she says it's a job for them and goes off in search of Sylvanas, letting Shandris and Malfurion manage Ashenvale and setting the Night Elves up in Hyjal.
Shadows Rising:
The book proceeds as before with some changes to the Night Elf plot - for one, Tyrande isn't actually present for the day to day running of the Night Elves, which has a major effect. We're given more of a glimpse into the power-up and its negatives - the more souls she ties to herself, the more she can draw from Elune power, but her physical body can only hold so much and those souls themselves have their aggression and grievances emphasized above all else, which takes a toll on Tyrande and makes her ill-suited for leadership. She's self-aware to let Malf and Shandris run the show, but with her going after Sylvanas, the night elves have trouble holding the lands they saved in the blitz, which includes Ashenvale and a newly Horde-less Hyjal. The first scene with Tyrande is changed to emphasize that Tyrande has specifically come back to meet with Thrall and company, and she immediately kicks them out, not taking any offer.
The second scene at the end of the book doesn't mean to involve Tyrande at all - she arrives at Stormwind upon hearing undead night elves are present to put them in the ground. Sira's characterization would emphasize her personal issue with Tyrande - how before she was the Night Warrior she killed her sisters to free Illidan at a whim, and now as the Night Warrior she's doing the same, putting her people in an overall untenable position long-term. She taunts Tyrande over this and for how at the end of the day whether she wins or loses doesn't really matter - their people've been reduced to a shadow of their former selves already and the best they can do is bring everyone else down with them and all she can do is secure the dead, not take care of the living. Tyrande nearly offs her, but she's stopped by Maiev, who takes Sira away, what with her being a Warden and under her command - the episode with the killing of the Wardens is fished up from WC3 and after nearly smiting Maiev herself, Tyrande walks off - she confides in Malfurion that she has a hard time differentiating between herself, Elune's voice and the voices of those she's saved from the Maw. She knows she won't last much longer and is afraid that Sira'll end up being right about her. She ends up vesting Malf and Shandris with permanent authority and goes off to finish hunting Sylvanas.
Shadowlands:
The fight with Nathanos from canon is kept as is with some exceptions - for one, Nathanos has goons that are swarmed by phantom warriors. For another, given his lack of appearances their final dialogue and the means of his death is changed - it's not even a clash, he's immediately fried when she enters the scene and after asking her questions and he taunts her she ends up for the first time using her power to grab souls not to save night elves but to essentially sunder his soul, keeping him as a trophy while she goes after Sylvanas. This way she gets the last laugh and a clean win and it's emphasized how the Night Warrior can circumvent the main gimmick of the baddie of the expansion, as well as showing how it's unnatural - if he shows up again then these bindings can unravel and his soul can be freed after she splits the Night Warrior power with others, and if he doesn't then this can be framed as a perma-kill/soul destruction. Either way it's unambiguous who wins, physically and otherwise.
In Shadowlands proper the main thing to change is her intro - Tyrande's power is the one emphasized, not Jaina's - the swathe cut through Mawsworn is done by her and Jaina credits herself to the Night Warrior, who's after Sylvanas in her tower. From then on, much like with @
Sondrelk 's idea, it's emphasized that a big reason we can even get a foot in in the Maw is because the Night Warrior is busy saving souls therein and seeking Sylvanas, but can't make heads or tails of the infinite tower and she's slowly burning herself out while she does it, hence Shandris going in to fish her out in the Ardenweald story. From then on, much of the same beats follow, but with the emphasis swapped around - the destructivity of hte power is noted, but so is the legitimacy of the grievances and how a person alone can't shoulder the burden, but many might be able to.
In 9.1, Tyrande does end up going to Ardenweald and gets to interact with her people, who don't just react to her but to the souls she's lugging with herself - it's emphasized how Tyrande's become very powerful through them but she's also keeping them from Ardenweald and a peaceful afterlife in doing so. Before she can make something out of it however and still bristling with power Sylvanas shows up - Tyrande fights her, they have a big cutscene and Tyrande flips her shit - she sees Kel'thuzad/Anduin about to nab the sigil - but dismisses it, hounding Sylvanas through the portal and into the Sanctum of Domination, where she's a major NPC later or even a boss. She takes Jaina's role in the Sylvanas boss fight, but ends up overcharged later and has to be dragged by the other Night Warriors into Ardenweald, even they though can't properly save her and they don't dare to since if they lift her powers she'd no longer be able to keep the souls she's saved protected and they'd go to the Maw.
Instead, the ones to bail her out aren't the new characters but Shandris brings volunteers, night elves alive and dead who'll agree to carry the burden with her, splitting the Night Warrior power-up between herself, getting the eyes and moon-related powers, specifically on loan until the Jailer is finished. Just as Tyrande/Elune saved the souls of those in need, now the living chip she helped protect chip in to protect her, getting a cool long-term cosmetic feature. Further on in the expansion, once the Arbiter is fixed they'd let go of the souls and return them to paradise, but the group sharing the power would end up keeping a version of it and be able to induct others who'd voluntarily stick around as vengeful spirits protecting the night elves.
tl;dr:
Rewrite is premised on keeping all the core events - same people have to live, die, overall expansion plot can't change, can't swap locations or factions, no new cutscenes/cinematics can be added, timespan must be the same. From this come the following:
-> The Night Warrior allows saving souls as vengeful spirits who buff up the user, the madness is a result of sharing a mind with them and the increasing power of Elune.
-> All undead night elf shenanigans occur prior to Tyrande becoming the Night Warrior, after that her simply existing draws all fallen night elf souls to herself and ups her power, keeping them from the Maw. This means just by existing and adopting the power she already weakens the villain.
-> Tyrande's intro is her stopping Nathanos from raising a bunch of night elves and killing him, only to have him raised by a valk later.
-> Tyrande is the commander in the Alliance version of the Darkshore Warfront, which takes place canonically after the Horde one and ends with a victory in 8.1
-> Night Elves fight for Ashenvale off-screen in the 8.1.5-8.3 stretch and reclaim it off-screen due to the Night Warrior sometime there, which is fleshed out in Shadows Rising. This is done largely by Tyrande and they can barely keep it on their own.
-> Shandris is charged with leading volunteer forces and doesn't second guess her mother but instead wants to help her out
-> In Shadows Rising Tyrande is ready to kill Sira but is stopped by Maiev who takes her off as one of her own instead
-> Tyrande's issues are a mixture of the physical toll on her body and the mental toll of hosting a goddess/vengeful spirits
-> Tyrande killing Nathanos has her seal/destroy his soul to get a win out of it
-> Tyrande is a major distraction for the Jailer and the reason we get a foot on the door in 9.0
-> Tyrande appears instead of Jaina in the 9.1 raid vs. Sylvanas
-> Instead of Tyrande chilling out because of Shadowlands-exclusive characters, night elves end up splitting the night warrior power between herself, giving the night warrior eye customization option and possibly the paladin class.