I wish Paper Mario 2's Audience would make a return. It added a nice randomness to some battles than only got more exciting as your 'stage' got bigger. Worrying about stage pyro, hoping a Shy Guy ends up knocking down a light on the right guy, and wowing the crowd with STYLISH!! moves for more star power. I'd love to see it come back and be expanded upon.
RudeCubes's comments