There isn't a single fucking aspect about Mario 64 that I would change. All I ask from a game is that it be intuitive - figuring out how to play something is half the fun. And although I wasn't ever able to %100 the game, there was never a time in Mario 64 where I was confused as to what to do. Stuff was hard as fuck, of course, but there's nothing more frustrating than being totally in the dark as to how to make a game stop eliciting howls of protest from you.

Obviously the above doesn't apply in the slightest to the Fire Pro series.
 

See, I'd probably change the camera and jumping system. I don't know if you ever got to play Mario Sunshine, but the controls for jumping were so much more intuitive. It made some things easier to do (I was never able to get that one star in, I think, Cold Cold Mountain because I could never do that triple-jump-off-the-wall move), but it also felt a whole lot more natural and forgiving. But yeah, Mario 64 was superbly designed.

- Dan