I quite like the idea of charging people for superficial stuff. I'm sure you could find a tonne of people willing to pay a small amount of money to play through your platformer as Michael Jackson or whatever, and for the time it takes for you to do an extra spritesheet it's a good deal for everyone involved. Still like I said, small stuff like that really needs a standardised payment system - once you get to the stage of small purchases, the inconveinence of actually paying is costing more than the purchase itself.
I was thinking about having adverts for new features which automaticall appear when someone loads the game for the second time or whatever. That way, you're already restricting the people who'll see them to people who like your game anyway, and you're not going to get OMG VOTE ZEROed by people who get scared away by that sort of thing.