At the start of the expansion its mostly war between alliance and horde combined with helping allied factions with their own antagonists but later it will most likely still have a main antagonist at the end, since Jaina and Rastakhan are official alliance/horde members now the chances of fighting them are very low especially as endgame antagonists, with Jaina's new model she pretty much returned to stay. The same can be said with the other faction leaders they don't really have potential to be the main antagonists of an expansion, even in Mists of Pandaria where it was the closest one to fighting a playable faction leader we still basically fought the old god Y'shraaj at the end. Since Queen Azshara is a boss in the 2nd raid of the expansion the most logical would be to have N'zoth as the last boss. Apart from Azshara, N'Zoth and the Void Lords there aren't any other major enemies that we haven't fought yet if we exclude Rastakhan who joined the Horde. And it's still a bit early to waste the Void Lords in this expansion so N'zoth is pretty much the only choice.