This is an excellent idea.
I think it would be cool if once you got to 50 there was a way to get one or two more signature powers. It would give level 50s something more to work for and such.
Also, I'm not really (or at all) educated on the programming of MMOs, but why couldn't you just have a create-an-animation thing? Like in the Tony Hawk Pro Skater games. >_> Just string together a bunch of small movements/particle effects or something, like a macro?