At the moment I am rereading the entire series (for the oomphth time now...) and I get new character ideas about every 3-4 pages (and forget about most of them about half a page after that). One idea stuck with me though, when I read about organ donation in one book:
A guy, born with a weak heart muscle and no magical power, doctors had not much hope for him living too long. However, around his 12th birthday he finally receives a donor heart, from a white council wizard (though nobody knows it at the time). From that point on his recovery is miraculous, thanks to all the perks his heart brought with it (Wizards Constitution and the rest of the wizard kit).
To be honest, I don't have much more beyond that point about the character yet, but I really liked the idea of someone getting his powers from a donor organ, and I thought the heart was most fitting.