I always reconciled that you have three possible versions of the Scourge --
"Bad" -- a Scourge controlled passively by an otherwise idle or inactive (i.e. frozen) Lich King. Granted, this is still pretty dangerous, because this is what you had at outset of the Third War.
"Worse" -- a Scourge controlled by an active and ambitious Lich King directing and controlling it toward world annihilation, but with a plan (this is Arthas from the time he wakes up in the cinematic, basically, until he is destroyed)
"Worst" -- the Scourge with no Lich King, just a frenzied uncontrolled zombie apocalypse.
Killing Arthas in favor of Bolvar was an effort to go from "worse" to merely "bad", although with some hope that Bolvar can just render them idle altogether. That's the only way I could make it make sense to myself. Fanwank is a powerful thing.
i would go back in time and kill azshara before sargeras noticed the reckless use of magic on azeroth, thereby preventing the legion, the orcs, the scorge, the naga, the bloodelfs to ever happen.
can't say it wouldn't happen eventualy though, and if it happened, the nightelfs would probbaly never have joined alliance with the humans. But the nightelf would be imortal, so who cares?
perhaps the trolls would still be a great force, with no highelfs to kill the amani trolls, no scourge to slaughter the Drakari, no shattering of the world isolating the Farraki trolls.
and also, with the majority of the orcs still on draenor (outland), they would probably have slaughtered all remaining draenei long ago.
and without the orcs, perhaps the tauren would be extinct now, seeing as the orcs saved them from the centaur.
so much changing just by killing 1 person
I would disguise myself as a Stormreaver clan member and inform Doomhammer of Gul'dan's treachery before he sailed to the Tomb of Sargeras. Doomhammer would slay Cho'gall, Gul'dan, and any of their loyal followers and would no longer split the forces in half. The Horde would destroy Lordaeron like Stormwind and continue on the war path until all the kingdoms had fallen (Human, Elven, and Dwarven).
Sure, there are Lichs and Necromancers and what not commanding small sects of the Scourge...
...But even they follow the Lich King. All forces of the scourge that are undead are literally part of the Scourge. It's like a chicken without a head otherwise: it'll thrash around and ruin EVERYTHING in its path until itself and all else is destroyed.
I loved the fact that killing the Lich King wasn't so simple.
Dunno why you have such a gripe with it.
And don't come and say "It's bad writing."
It's Blizzard's game.
They do what they want with the story.
To be frank, get over it.
Reversing the cataclysm
How? Imagine that to defeat the Burning Legion is needed the power of the Five Flights... so we should travel to the past to prevent Deathwing was corrupted.
I would fix Kael'thas going whack job. It always bothered me, despite my love for TBC.
I know Blizzard did it just so they could give some Draenei/Elf Conflict and position them inside the horde more easily (Though Garithos on his own had been reason enough).
But oh well, it shows for sure that the exposition to fel magic corrupts absolutely. No matter the intentions of the wielder.
Put into motion plans that would cause a domino effect, leading to Saurfang the Younger becoming Thrall's right hand in Northrend while Garrosh is left to rot at the Wrathgate.
Stop Ner'zhul from meeting Kil'Jaeden.
Delete Cataclysm. Might seem like a stupid and overused thing, but I have my reason for that:
Deathwing was among the best, if not the very best, villain that Warcraft has ever had, and to see him get thrown away like the stupid minion that he was sucked major ass. Deathwing is the one who is supposed to be in charge, he was once known as one of the most threatening presences in all of Warcraft history, and yet he was just octupussified and thrown into the trash can without hestitation.
While I do think that Wotlk had the worst lore for bastardizing all the character involved, Deathwing has always had more potential than the Lich King imo, and to see it thrown away just like that kind of hurt.
Bottomline, Deathwing shouldn't ever have been in WoW. He should have been saved for something like Warcraft IV where he could truly shine. MMOs have always been mediocre at telling stories at best, while RTS games tend to do it very well. It all has to do with the fact that the player can't die permanently. Had Cataclysm been the main center of events in a Warcraft IV, then it would have been genious, but the fact that it's an expansion for a MMO fucked it up.
Overall they should also just have dropped the idea of Deathwing being N'zoth's minion and being in the MMO. Deathwing acting on his own would have been way more threatening, showing that he doesn't need masters, he is a master himself. Too bad it became a tentacle expansion rather than a black dragon expansion.
Change Bolvar/Saurfang's fate. Bolvar would become a DK like Saurfang. Alliance fights Saurfang, horde fights Bolvar in the raid. Both DKs come to their senses and rejoin their respective factions after the raid wins the fight.
Those two characters are too awesome to be killed off the way they were. Not sure who that'd leave to be the new Lich King though...I guess Tirion would've ended up doing it after all.