I could see Sylvanas being the boss of the next expansion. Not this one. This expansion is about setting her up as a villain but we've got N'Zoth and Azshara to deal with. The next expansion will be full on Sylvanas as the big bad from start to finish. Think of BFA as her Warcraft 3 and the next expansion as her Wrath of the Lich King.
Since we seem to revisit themes a lot we haven't really had a undead themed expansion in a very long time and that might be their big attempt to draw people back in to the game. Sylvanas is one of the biggest characters there is.
I could see the next expansion to be a war between the dead and the living and it would be cool if it had a cataclysm style revamp only instead of Twilight's Hammer and dragons it was lots of undead.
And I think that could be what brings Bolvar back in to the story and maybe even Bwonsamdi. A death themed expansion.
What Sylvanas will realize is that the real problem is the living. The living will never see it her way. The Horde are all turning on her because being alive is what makes them weak.
And maybe this could finally be the expansion that Anduin actually gets to shine. Not just at the start but throughout the whole thing. As for Horde, maybe Liadrin, Baine or Calia (who could replace Sylvanas in leading the Forsaken that stay loyal to The Horde) will be their light themed hero.
I think what happened to Derek Proudmoore and Calia Menethil could be planting the seeds for the next expansion. And also, Taelia being brought in to the story is obviously setting up something with Bolvar who is also undead.
And just a little side thing: it would be cool if the final battle against her took place in the burned down Teldrassil. That whole place could be revamped as a zone or two.
Anyway, what do you think?