There are going to be a number of question marks for the game after a while.
The first big contnet expansion, The Burning Crusade, dealt mainly with the Orcs' homeworld, Outland (or Draenor, whichever you prefer), and its final boss, originally thought to be Illidan Stormrage, was Kil'jaeden, a demonlord of the Burning Legion and second-in-command to said Legion, bowing only to its lord, the fallen titan Sargeras.
The second (and current) expansion, Wrath of the Lich King, deals with Northrend and its "leader", the seemingly-omnipotent lord of the dead, the Lich King, Arthas Menethil.
After WotLK, where can the series go? Well there's the ethereal realm of the green dragons, the Emerald Dream. Something has been corrupting the dream for some time to the point of there being four raid bosses in old-world Azeroth called the Dragons of Nightmare, all of them green dragons to add to that. Question is, where would there be a final boss? We don't really KNOW anything about the Emerald Dream except it's where the druids go to strengthen their ties with nature and that something's corrupted. There's also the Maelstrom, where the Well of Eternity imploded and left the Broken Isles, where the last boss could be the queen of the naga, Queen Azshara. Another potential raid out there is the Tomb of Sargeras. But again, what else is there except that?
After those two concepts are worn dry, I really think that might be the end of the game. There's really nothing else in the lore they could try, unless they want to try and create something else. And with the lore of the game already established, sounds a little stupid.
