Well we have certain leads to what it could be from here onwards, but you can never be sure.
1. Chris Metzen stated at Blizzcon 2011 that Illidan would make a comeback. Illidan is a STRONG enemy of the Burning Legion and has worked his whole life to eradicate them in any way possible.
2. Wrathion, the uncorrupted son of Deathwing is concerned with what is happening in Azeroth, mainly the war between the Alliance and the Horde, and reminds us, as players, that there are BIGGER enemies and shows a Comet hit Azeroth in the meantime.
But even though a Naga expansion wouldn't be that far-fetched, and it actually would enable us to see some new mechanics (for example underwater-fighting).