With all of the talk of what we will see for World of Warcraft's next expansion, wouldn't it make more sense to "wait" with the Burning Legion from a story point of view? If you look to Azeroth you will see we have Azshara, and the Old Gods still fumbling around on this ol' piece of rock, and before we venture off to a new world, it would make sense to wrap things up here at home. Do not get me wrong, personally, I would LOVE to fight the Burning Legion once more! I find their story, parts of it retcon or not, interesting and the wild landscapes are refreshing.
As we know Sargeras is the biggest baddie of them all (from what we know), and instead of just transitioning from the story of Pandaria straight to the Burning Legion without wrapping up other events on Azeroth, this would only belittle the foes we currently share a planet with. My theory is that Mists of Pandaria is only the beginning, in that it is slowly setting up the next Burning Legion expansion. This can be seen with Wrathion's various doings; Azeroth being attacked by Infernals, "Uniting" Azeroth's races...etc. As well as the future "Army of Light".
As much as I would love to explore other planets, I just have a feeling next expansion would cover the following...
- Queen Azshara as the main villain
- Old Gods doing Old God stuff
- Zul & the Zandalar FINALLY being wrapped up/explored (This could make sense if a certain Troll were to rise to power within a certain faction...)
- Broken Isles, but more importantly the Tomb of Sargeras
As for an idea of what they will be doing, I suggest Queen Azshara will be calling out to the Burning Legion once more, and this will only strengthen the return of the Burning Legion for a future expansion making that much more epic!