I haven't seen it yet, but an adaptive fin would be the coolest. It could change geometry depending on how much down-force you needed. And, even better, it could be covered with "electronic paper," which is able to show messages in black-and-white. So, sometimes you could have advertisements (when you felt really rice-like), and other times you could just make it black or white, or perhaps even checker-boarded.
But I would say that the one thing that the M5 needs above all else is a really good pair of wheels---not any of this BBS crap, but something from Dayton:
http://www.daytonwirewheels.com/
Your tail fin could sometimes say, "don't hate the player, hate the game."
Linear Mode
