First of all, 343 didn't create an entirely new game, so many of the things that are "better" to you, was created by bungie in the first place, especially considering the fact that many employees stayed with microsoft to join 343 when Bungie bought themselves out.
Secondly, how do you think it would be even remotely possible to create an MMO like WoW, on Xbox? Have you ever played WoW? because there's too many buttons on a keyboard to transfer onto an Xbox controller. If you mean just an MMO on the Xbox where many people can group up to complete an objective, yes that would be cool, but very difficult with the engine required and the Xbox now couldn't do it, maybe next-gen consoles, maybe. And if you mean just an RPG fantasy game for Xbox, there's this wonderful game called The Elder Scrolls V: Skyrim, it's pretty awesome, not to mention countless others like Fable, Dragon Age etc.
I simply wanted to clear up that thought about WoW on Xbox.
Back to the topic, I don't think that they should make an MMO for Halo, it seems like it would shift the series in a new direction, and that isn't what I want, of course, my humble opinion doesn't matter. While i have no doubt that it would be succesful, (that is, more succesful than The Old Republic) but Halo is simply an FPS game built around a community, and i feel that the game would leave a lot of the fans behind and maybe get a new following, which is certainly not what i want. Although, if they did make an MMO, I would have no doubt that i would try it out.