It all depends on the conditions. Is Samus allowed to use the morph ball and, by extension, bombs? What about missiles? Is Mega Man allowed to use all the weapons he's obtained from the bosses, or what? For that matter, how much ammo is Master Chief allowed to carry? What about grenades? Is he allowed to drive the Warthog, or any other vehicle?
But back on topic, I'd like to be a robot that transforms into an Ancient Egyptian pyramid, and vice versa. I'm not sure if that's from any game, but it still sounds awesome.