I think the Win/Loss system based on Player is a good way to do it. Early on I imagine there being a lot of movement, but over a long period of time I think people will seperate from the pack and be at the top with others scrambling below. It could lead to things being stagnant.

Versus say, a Win/Loss based on character. Then at least certain characters could be retired after a time and allow for others to come in and fill the gap. Maybe something like a Hall of Fame?

Just brainstorming.