Hey all, this topic is all about an expansion many have speculate about, and what it could possibly contain. I'll go through the typical things you should come to expect, and maybe a few extra bits.
PLOT
So what would warrant a return to Outland? The only theory I managed to raise involved helping to cleanse any remaining threats to the ravaged planet. There could be old villains that were forgotten about since we ventured there in 2007 that weren't properly exterminated. I don't think the Burning Legion should return to this world, as quite frankly, it has suffered enough. We could have a short story involving some stupid nutter trying to open a portal that a few lesser demons enter through, but this would end. It could be over the course of an entire zone if the story warranted it.
Another lingering thread is Baron Sablemane and his uncorrupted black brood. Wrathion arrives through the Dark Portal, and learns word of his brother. They have a mental and physical battle, and hopefully come to some truce/understanding. These 2 guys have potential, and to kill them off would be sloppy writing and anticlimactic.
I think there wouldn't be enough to base a whole expansion back on Outland, so the following would instead continue the ever-present Burning Legion theme:
A NEW PLANET
We haven't ventured much into space, so a new planet would be interesting to discover. I have no idea what it could be, or what races inhabit this place, but the adventurers discover it and help to defend it from the onslaught of some of the Legion's commanders/armies. How would we know about this place? Maybe The Exodar receives some distress signal, and now the Draenei muster their troops instead of sitting around, and aim to save a world (bad memories of seeing their homeworld become corrupted spur them to action this time).
The alternative:
As we have seen from the map for Draenor for WoD, there is the Ogre empire continent in the SW corner. Maybe there also other continents on the other side of the planet, and some of these remnants are discovered in the Twisting Nether, still inhabited. We probably get there via portal or inter-dimensional ship, and maybe Turalyon and Alleria are there! (Hey, we need them back). Again, some Legion activity could be on these potential zones to continue the theme.
How to tie up the end of the expansion:
As with The Burning Crusade, the plot and theme is brought back in the final patch (Maybe including the penultimate patch as foreshadowing), as some evil minds have attempted to contact the Legion, and use a portal to let the Legion's armies through, although hopefully it wouldn't be too derivative of Fury of the Sunwell.
RACES
Seeing as we came back to Outland, we finally get Arakkoa for the Alliance and Ogres for the Horde. They do seem like the most logical choices. What race/class combinations would there be?
Arakkoa
Druid (I imagine they'd be taught by the Night Elves)
Hunter (They could use those kaliri)
Mage
Priest
Rogue
Ogres
Hunter
Mage
Shaman
Warlock
Warrior
LEVEL RANGE
I'd go with an extra 10, and instead of the choice between two starting zone you could alternate between planets if you got bored (If that'd work!).
OTHER INFO
Dungeons
7-10 at the start so more could be added in future patches. Again, the dungeons could alternate between worlds.
Bosses/Raids
You'll have typical Burning Legion commanders/lower ranking officials for some raids, especially in later patches. Maybe some new races on a planet-those considered local threats (e.g. The Thunder King). There could also be non-Legion outer space threats that seek to invade either the new planets, or Azeroth. One raid could even take place in space, like fighting on an actively moving ship. A phase of a fight could involve using vehicle aerial combat, like against Malygos with those drakes.
So what do you think of my ideas/theories? Is there anything missing that you would like to add to keep the expansion interesting? All constructive criticism is welcome, just keep it civil.