I thought along the same lines as you guys who were thinking Ryuichi in a bunny outfit or Shuichi in his dog one. As aarin said, it's gotta fit the site because it's a representation of it.
I also think it'll be hard to do because the way I picture a contest is this:
All the creative folks on this site run photoshop wild, and what we end up with is the equivalent of a signature with a well-known anime figure. I really think it would need to be an original character, a really well-drawn one, and then what do we end up with? No offense, guys, but I don't really think it's getting any better than heise. At that point, it's pointless.
I'm still on the nay side of this and totally against having a little chibi or a shota or something like that.