How BfA's story could potentially end with as much grace as possible.

Now that he's free, Derek Proudmoore (who should realistically be nothing more than a handful of random bone fragments at this point, but I digress) overcomes whatever brainwashing Sylvanas did to him and reunites with Jaina and his family, proving to everyone once and for all that the Humans are willing to tolerate and accept the Forsaken. The various leaders of the Horde finally decide to depose their current Warchief to make peace and encourage undead/Human relations.

This makes Sylvanas very upset. She now decides that the Alliance must be destroyed immediately and she *will* make that Little Lion bow to her, dammit. So she gathers up those still loyal to her and begins the final raid of the expansion: Siege of Stormwind.

So now the Alliance and the Horde join forces to stop that crazy ho. We slaughter all of her remaining loyalists, murder Nathanos in the face like he so deserves, then walk into the throne room and beat the shit out of Sylvanas until she finally submits (because let's face it, Blizzard won't let her die off that easily).

The Forsaken choose a new leader that will help bridge the gap between them and their living counterparts (probably Derek or Calia Menethil). Sylvanas escapes custody (but of course), travels to Northrend, murders Bolvar in his throne where he's still frozen solid, and the next expansion begins:

World of Warcraft: Wrath of the Lich Bitch.

I would not be surprised at all if this is what happens.