This is my speculation on a possible story development in BfA and possibly next expansion.
Until recently, I was sure that 8.1 will be faction raid, 8.2 - Azshara raid, 8.3 - Ny'alotha raid. And still it is really really probable. But, there are several hints that point in a different direction.
First of them is... Uu'nat the Harbinger of Void dungeon journal. Look carefully. All his abilities and names of the phases make full impression that we are fighting some kind of N'zoth's avatar. This makes an impression that we are having a repetition of N'zoth fight for the expansion. And I find it a bit strange that we would get both this fight and N'zoth himself as the main boss. Like, we had Avatar of Sargeras in Legion that utilized weakened versions of Sarg's spells, and no Sargeras fight in the end.
But this, of course, is not the main reason. Actually, we have a real alternative to the N'zoth threat.
Death.
This theme is heavily hinted throughout all the expansion. At first, we have something that interferes Vol'jin's death visions and forces him to appiont Sylvanas. Which, ultimately, causes war.
On Kul'Tiras, Death tries to subdue Drustvar. All the Drust storyline is entirely Death-themed, and Gorak Tul, when defeated, says that he awaits us in Shadowlands. Yet, later he is apparently killed by us and Jaina, but I'm sure they can conclude this line later again. After all, we seemed to kill Helya in her own domain, and now we get notes that she is alive and gathers strength. That may be the case for Gorak Tul as well. At least I'd like to see him as a raid boss. Still, he tried to brainwash and capture Jaina, which could ultimately give Kul'Tiras in the hands of Death.
In Zandalar, Death line is represented by Bwonsamdi, who mentiones that he has a boss. He succeeds at where Gorak Tul failed - he has Zandalar in his hands, and not just Rastakhan, but all his desendants. And in 8.1 raid he is in fact the endboss of the raid - he takes heavy part in the fight and likely departs when Rastakhan is dead. And Death theme again pops up openly.
Sum it up: we have someone powerful from Death realm who made the Horde what it is and the whole war. War where Horde resurrects fallen Alliance warriors and, most likely, binds their will. More to that, a loa of death, who lives in shadowlands, takes in charge over Zandalar and simultaneously king of Drust, who lives in Shadowlands and wields death magic, tried to subdue Kul'Tiras. And what if both Bwonsamdi and Gorak'tul have the same boss? The one who Odin made a deal with, one who is formless, constantly shapeshifts.
Trolls know him as Ueetay no Mueh'zala, or simply Death. His titles are:
God of Death
Father of Sleep
Son of Time
Night's Friend
He speaks a very cryptic phrase: "Not yet, Son of Thorne. Not yet. This is not the day. The day comes. It comes. But Mueh'zala will not engage you here or now. Our battle is yet to come, yet to come... But it will come, child. It will come. And if you lose that battle, Mueh'zala feasts on all of Azeroth. All of Azeroth. All of Azeroth. All of Azeroth..."
See? It seems that we have something really big here. And Azeroth now is really weak, and that's the best opportunity for Death to feast on her.
So, like Fel aka Disorder is enemy to both Light and Void (and Arcane), and Sargeras wanted to kill Azeroth to prevent her from corruption, so may Death be enemy to both Light and Void (and Life), and be planning to kill Azeroth to prevent it from corruption.
Now, that is my version of the further plot.
8.2. - general battle between A and H occurs, on the seas. Just as in the Ogmot diary, "Last night, Ogmot dreams of great battle. Two fleets bashin on ocean of blood. Shadows twistin beneath da ships, risin. Risin. Ogmot so happy watching the carnage!".
So, when the two fleets clash and fight, Azshara and her army appear and get them both on the bottom. And between Kul'Tiras and Zandalar is the prison of N'zoth. So, since Azshara is the Gul'dan of BfA, she will cast a spell that, using all the massive bloodshed, finally opens the prison and sets an Old God free. We try to attack N'zoth, but he effortlessly wipes the seafloor with both armies and swims away.
Let me also remind you something. Leader of the Horde takes crazy and immoral steps to win war at all costs. Other leaders disapprove, one of them goes defiant, Warchief sends assassins to kill him, but they fail. And that rebel becomes centre of opposition and new Warchief. I believe, when they said they won't repeat SoO, they mean this: after losing in 8.2, Sylvanas' position is weakest. Her own land is captured, Baine and even Gallywix believe she is crazy, Saurfang is coming for her. So in 8.2 he comes and they together force her to step aside and lay down the mantle. At that point, she departs, not willing to be killed outright.
Saurfang becomes Warchief, he and Baine ask Alliance for a ceasefire since both factions are heavily wounded, N'zoth free and the Naga are trying to conquer all. We get Nazjatar as big new zone and deal with Azshara, who understands now that N'zoth will not help her out - she's now out of use for him, having done what he wanted.
8.3. While A and H are occupied with Azshara, Sylvanas, banished, flees to the Shadowlands. There, she faces the formless creature that led her along the way. Now it promises revenge, power and immortality. We have a whole army of Death popping out of nowhere, probably using Azerite as portal to open the way between realms. Here we have:
*Sylvanas loyalists - maybe Sira and Delaryn. I'm sure they were risen not just so. As well as Derek Proudmoore.
*Helya and her Kvaldir
*Bwonsamdis' agenda - probably
*Gorak Tul (if he's not dead) and the Drust.
*Arguably, but Lich King. Yes, he says that he's a keeper of balance, but what if he's lying? Bwonsamdi also sais he's against Sylv, but his boss may be thinking differently. And about Lich King we have some big hints that his plans are not entirely noble. Also from Ogmot: "Bound by dat throne? No. Free. Free!". So that's at least possible that he stands up from the throne and Scourge joins the Army of Death.
And all that is lead by Mueh'zala or whatever his true name is. Also note, if Sylvanas is in the raid, she becomes a raid boss just like other 2 Warbringers characters. So, we have Shadowlands raid, Calia Menethil becomes new Forsaken leader, Menethil rules Lordaeron.
Surely, Shadowlands can be saved for next expansion, but in that case we'll have to deal with OG problem in just a patch. And have entire expansion themed around Death. WotLK-2? Don't see that coming.
9.0. - the Black Empire.
Having dealt with Death problem, we still have a nuisance - N'zoth being free, and no power on Azeroth can do anything to stop him.
And now, let's remember that Nozdormu will go "mad" and turns to Murozond, creating Infinite Dragonflight to prevent something. Now let's recall. "You crawl unwitting, like a blind, writhing worm, towards endless madness and despair. I have witnessed the true End Time. This? This is a blessing you simply cannot comprehend." And now ask yourselves - what is actually worse than to have all life burned out by Deathwing?
What is the one thing N'zoth will do once he's free? Break our puny kingdoms? No, he has a purpose. He takes his tentacles to Azeroth's head like a giant mindbender and create the Dark Titan. Yeah, that's more like a true End Time. Seeing this, Nozdormu begins his time play, because we have no chance to stop him now. Thus we can have new zones of the Azeroth of the past. Here is the main plot of next expansion, ending in Ny'alotha.
10.0 - with last major threat on Azeroth dealt with, we only have to deal with global war: Light, Void Lords and probably Sargeras. Which would hopefully be the last expansion.
Though, this is not the main theme of the post. I've presented my version of plot - in BfA we face Azshara and then Death threat, while N'zoth, Xal and Murozond are saved for 9.0, Black Empire - themed expansion.