With we beating the Legion on Argus, saving the titans and defeating Argus the Unmaker (Argus correpted worldsoul, or whoever he is) we will have squarely saved Azeroth from them for now.
Throughout the expansion we destroyed crucial parts of the Legion's army, generals and footholds. Now without Archimonde (presumably), Kil'Jaeden, the planet Argus, the titans souls, and this freaking Argus the Unmaker, it seems like the perfect time to hunt a weakened Sargeras and try to put an end once and for all to him and his burning legion, isn't it?
Illidan seems bent on fullfilling the Legion's demise, and if anyone played him in HotS, you know he sticks like glue to his target. Lore-wise, why wouldn't azerothians hunt Sargeras throughout the cosmos now?
So what do you think? Will "the hunt for Sargeras" be the next expansion? Or will we still face him this expansion? Or will we get a break from Legion next - maybe to deal with an Old God outburst- while Sargeras regains his strength and plot his next move?