As I prepare for my own take on Gamma World, which is set 30 or so years after the Big Mistake, I've drafted three new origins based on your character's age: Young heroes who were born since the Crash, Survivors who were young when the Big Mistake happened, and Old heroes who had full lives before the Crash and are now among the oldest people still alive.
They can all be found in my House Rules
. They're also posted on my blog
Comments welcome! I also still need a picture to represent "Old" characters, so if you have any ideas, speak up.