At this point, its all speculation, but heres how I see both of these possible expansions shaping up to be:
Naga Themed:
2 New Races -
Naga: A splinter group of Naga formerly under the control of queen Azshara breaks off and becomes loyal to the Alliance. They were formerly night elfs and now have to prove their loyalty and gain regain trust with the night elfs and the alliance. Could be a shakey alliance out of convenience too (sort of like goblins)
Ogres: The ogre tribes band together and oust a group of ogres they see as inferior to their clan. This ogre mound now ousted by their own kind turn to the horde to take them in. With Vol'jin now the new leader of the horde, the ogres are welcomed in and become an asset to the now shattered horde.
1 New Class (Maybe):
Tinker - Look to all the discussion surrounding the tinker and possible abilities.
Environments:
South seas, and oceanic. Would be hard to pull off as there are already so many beach environments in the game, but they could probably fathom up something out of thin air like they normally do and pass it off as content.
Big Bad Baddie Boss:
Either Queen Azshara, or the Avatar of Sargeras. One of the zones that could be in the game could be the tomb of sargeras as a raid zone?
Legion Themed:
2 New Races-
Etherals: Convinced that the burning legion now poses a much more imposing threat to their money cartel existence, a faction of Etherals form a union with the Alliance out of convenience. They other etherals remain neutral NPCs and are spread out around various locations in the Legion's home world.
Ogres: Read the same as above. The splinter faction of the ogres want to now prove their worth to the horde and show they are the dominant mound by entering the home world of the burning legion to help fight the true threat...
1 New Class-
Demon Hunter, OR Tinker: Both have potential for a burning legion themed expansion. Demon Hunter would be harder to pull off imo, but it could be done.
Environments:
Homeworld of the burning legion with all the various new goodies (art assets).
Big Bad Baddie Boss:
Sargeras himself.
Which expansion do you think will happen?