IMO, Battle for Azeroth should be all the elves break away and make their own third faction. Elven Supremacy!
Sylvanas is trying to start a world war, and Lor'themar is just like, "You know what? Fuck this, this is dumb, I'm out of the Horde." Then he sends Tyrande an emissary like, "Hey, sorry about your tree. That's not cool. That wasn't us. We have a well we're pretty fond of, we get it. We left the Horde because of it."
Then Anduin's like, "Hey so we destroyed Lordaeron, now's the time to get revenge for my dad and end the rest of the Horde too!"
and Tyrande's like, "Uh... no. Destroying Lordaeron doesn't fix my burning tree, I have a civilization to rebuild, I don't need another fucking conflict right now.",
and Anduin presses to keep attacking the Horde,
and Tyrande's responds, "Okay, I see how it is. Fuck you. We're out of the Alliance. I have to put my people first, we just lost our capital and our world tree, and all you want is more death."
Then the First Arcanist Thalysra is like, "Hey Nightborne, I'm your leader now! We're gonna join the Horde because Anduin's a dick and we need to make all the expansion conflicts even to maximize carnage!"
And then Ly'leth Lunastre is like, "Uh. What? You're retarded, take your Nightfallen and go join the Horde. We just had our city sacked by demons, then sacked by both the alliance and horde, then our entire chain of command was destroyed, and we only just restored the Arcan'dor and can leave our 10,000 year bubble dome. We have to get our house in order before we go involving ourselves in someone else's war we know nothing about and have no stake in either way." - and the rest of the city-dwelling Nightborne all join Ly'leth, and expel the Horde Nightfallen from the city to go share a dorm in Orgrimmar or whatever.
Then Tyrande's like, "Hey Ly'leth, I heard what happened with Thalysra. Sucks for her, but do you mind if we crash on your couch for awhile? We could sort out your problems in Val'Sharah and Azsuna for you, and all we need is somewhere to not be on fire and homeless for awhile."
And Ly'leth responds, "Sure, cool beans. Thanks for teaming up with Lor'themar and killing Elisande and Gul'dan for us actually - you're cool in our books. We'll get you some blankets for that couch. Speaking of Lor'themar, what's he up to these days?"
Tyrande: "Dunno, but he sent flowers after my tree burned, very thoughtful - I'll go check on him and let you know"
So Tyrande sails over to Silvermoon just to hang with Lor'themar for a bit, and thank him for the thoughtful flowers, and they get to chatting - and Lor'themar's like... "You know, I'm feeling a little vulnerable over here with the Scourge and the zealous Paladins in EPL, not to mention being ditched on a continent now full of Alliance, even if I'm not Horde anymore. What say we get the band back together and do like an... Elven Empire?"
So Tyrande winks and says, "I didn't want to say it first. We're in! I'm sure Ly'leth is down too - her people mostly just want some new people to talk to after 10,000 years, but they are also super racist against ugly non-elves, so we're really all she's got as options anyways."
And so the Elven Supremacy was borne, and then they genocided all the boring other races and lived happily ever after.
The End.