EXP 5: Burning Legion and Argus
Too much suggests this as our next destination, for it not to happen. The hints of a threat from outside of Azeroth; the nod at Turalyon and Alleria being needed for what's next; the 'divide and conquer' tactics employed on the Horde and Alliance factions during Mists of Pandaria.
EXP 6: Queen Azshara and the Ocean
There remains a lot to be said and seen beneath the waves of Azeroth. I think they purposefully left the main 'ocean-y' bit of Azeroth from Cataclysm as it would risk overshadowing the rest of the content. There are many islands yet to make their way into the game, and Azshara has been looming in the shadows for years.
EXP 7: The Other Side of Azeroth
I don't think Blizzard will be ready to return to the Legion theme quite so soon after EXP 5 and 6, so I think they'll want another 'back to our roots' expansion. The other side of Azeroth is rumoured and has been debated for years. Sailors are said to disappear beyond the shores of the Eastern Kingdoms and Kalimdor, and there could be room for another continent back there. If there even is a 'back there'. I'd imagine this as an expansion all about the essence of WoW: exploration and the discovery of new lands.