Some people come with the theory Vol'Jin is not really dead and it's all part of a master plan so he can work in the shadows, fooling the Legion and working in the shadows like he is used to.

From a lore point of view it makes sense... he pulled this trick before on the books, and the Legion is full of spies and infiltrators so it would be a nice way to outsmart them.

More importantly, I don't think Sylvanas "fits" being Warchief flavor wise.

This also would make for a huge comeback when Vol'Jin pops from the shadow and kick Gul'Dan ass, giving him some much needed spotlight.

We may be wrong of course. Maybe Blizzard wants Sylvanas on the front for whatever reason and Vol'Jin was collateral. If this is true it sucks to be horde because they'd have very few relevant characters left.