In my honest opinion, their will be at least two more expansions before we face Sargeras, Kil'jaedan, and The Burning Legion.
There are still a few unexplored parts of Warcraft, and lore not being touched on as a whole, important characters, i.e.;
Queen Azshara,
Xavius,
N'zoth,
Nazjatar,
Ny'alotha, to name but a few. I think they'll explore these areas first, i.e. Corrupted Emerald Dream, with Xavius and the Old God N'zoth responsible for the corruption, as well as Azshara and her naga, Nazjatar, Ny'alotha, etc.
Al the above mentioned ties in to the Burning Legion in some way or another. I would think they tell that story first.
After all, the way I see it, The Burning Legion is the biggest threat to Azeroth, and the Cosmos. I expect them to leave the ultimate baddies for last, but then again, I may be wrong.