I think teams should be able to designate a captain just like they do numbers... put a "C" symbol next to their picture, but don't have the captain role actually affect the game. Any time, like on the front page, where a team is listed with a picture of one of their players, it would be their captain. It would just be cool to see who teams pick to represent them but it doesn't need to affect gameplay.
Interesting. It sounds like it could maybe be a Supporter feature?