I like the idea (and wouldn't mind it if that happened to Stormwind either, it have just got shit lately lol, and on the realm Im playing on Horde own it anyways xD), as by logic I dont see why Alliance wouldn't do something about it while they actually there. Not after Theramore, not after Southshore and many other things, those things have just boiled it up. Actually going to be a bit sad if Blizz make Alliance forget and forgive that (especially now when they actually aim for more Horde vs Alliance).
Though I dont see why Alliance help Horde out from the start; just let them kill each other off and then just rofl-stomp whats left when either Garrosh or rebels are left standing. Horde get Garrosh killed and Alliance get Horde killed, win-win! 8D