one thing I'd love to do is give a player the ability to create something in my game. for instance, if a player wanted to design a platform for a fighter game, I'd like it if they could draw something in paint or whatever graphics program and then import it into my game. the animation could then be applied to a pre-existing actor.
also, I'd like someone to be able to take some data from one game, and apply it to a completely different game (that has all the same variables, of course)