Seriously though, Wrathion is being built up as a far grander character that will help thread together multiple expansion stories, while also serving as a "third faction" that both Horde and Alliance players find themselves aligned with. I see the "war" in warcraft shifting from Ally vs Horde to Azeroth vs Burning Legion for an expansion or so to shift power struggles and rebalance some of the dynamics.
Think of Burning Legion as Nazi Germany, with Ally and Horde as US and USSR, in no particular order. They will need to work together to some extent, but will be distrustful and will try to set themselves up to emerge as a hegemon. If Blizz ever gets around to making a Warcraft IV, it would be fun if it occurred immediately after a large power shift to make way for some new characters and hopefully to build the beginnings of compelling storylines that could be further fleshed out in WoW expansions.