Its just makes way too much sense.
Varian gets engulfed in fel magic by Gul'Dan, possibly transported to someplace so he can get the demonic thing going. Last boss in expansion he gets revealed as the final boss.
More proof?
Blizzard likes to balance scales.
Garrosh was a final expansion boss who was corrupted. Now we need the Alliance version of that fight.
More balancing?
Tyrion dead, Varian dead. Maybe you can make a case for Arthas but he was something entirely different in the lore.
Vol'Jin dead, Garrosh dead.
Panda and Draenor were very Horde based expansions, Lich King and this one seems to be heavy Alliance, of if you dont want to count LK, Legion and the next expansion could be the heavy Ally story, as Jaina is going to have her own expansion soon enough.
Final thought about Varian being the final boss?
Anduin is still kinda wimpy, a final fight against his father who turns good at the end of the fight to kick him in the pants could be what he needs to become a great kings, which of course he will become.
Makes sense?