I personally believed it would possibly be an Old God raid, but I'm free to admit when I'm wrong.
Wrathion, in the final quest for 5.2's Legendary Questline, says the Pandaren Campaign is about to reach its climax.
And since we can safely assume 5.3 will be another Lore Patch, it'll probably involve the Horde Rebellion's uprising, and the Alliance staging an attack on Orgrimmar.
So what do you think. 5.4 is Siege, or maybe another raid and then Siege?