For those interested in more Gamma World origins, there is a pretty good effort on the WotC forums to compile user-made origins. You can check it out at http://community.wizards.com/go/thread/view/75882/26075109/Custom_Origins_Compilation.
Some of my favorites concepts from the thread are Amphibian, Avatar, Cosmic, and Dimensional. Amphibian makes me want to see some Battletoads characters. Avatar nicely encompasses the idea of being a willing vessel for a powerful entity, but it could be pretty easy to reskin as being involuntarily possessed. Cosmic is for beings composed of only light (think living stars) and has a nice set of powers to support the theme. Dimensional is a nice wacky concept, but the mechanics for it seem like they might be a little overpowered.
My Two-Headed origin is also included in the second compilation pdf. In addition, the first compilation pdf includes alternate takes on shark and dinosaur origins (as Sharkoid and Saurian).