I was thinking about different types of players on both teams. Maybe defenders who stay behind and guard a small part of the pitch, while attackers chase the ball. Maybe an idea of a "Season mode" where you buy and sell players in different positions (maybe with different stats, affecting how they play). Or if it were to go in a more "puzzle" direction, I could see various scenarios with interference due to penalty walls, streakers, hooligans in the pitch and so on.
It seems like a very fluid concept for a game, and it appeals both to sports fans and people who like physics based stuff.