Supporter is great at the moment, and Private Leagues are a big part of why. But there are a lot of supporters that aren't in a PL. My suggestion is to allow Supporters that aren't currently in a PL to schedule a match against another Supporter who also isn't in a PL to a challenge match. It will basically be like a PL game, but you can choose anyone to play against, and there is no complexities such as number of rounds, waiting for a certain number of players etc. It would occur on friday night just like PL games do.
This allows supporters that aren't in a PL to still have the advantages of a PL and play against their friends whenever they want rather than having to create a PL to play against a friend.