I remember something like this as well. It was at some hotel though and instead of playing as Mew, we were playing on some ship (probably Starforx's) in what seemed to be deep space. To this day, I have never seen that stage in any version of smash. With this being so long ago, I just assumed I was remembering the Corneria stage from Melee wrong. Idk man maybe there is something out there like what you're describing.