
I love the play with scale.
I loved it in Psychonauts, when you were Goggalor in Lungfishopolis, I loved driving my match box cars through sand castles and toasters in Micro Machines and I loved the level in Serious Sam 2 where you shoot your way through a backyard with giant cigarette buds and mushrooms.
I would love a RPG or FPS game that focuses on this aspect instead of using it as a gimmick. It would be interesting to morph from the size of a mite into something tree high all within the same environment and be able to do it at will. You could restrict the game world to one small island, a mansion or even a single room and it would still be big enough for hundreds of hours of exploration. I have no clue how that could be implemented, just thinking of UI and physics restrictions but it could offer a very different experience from today's game worlds that always brag with being X times bigger than X's game world. Instead make the area you can explore deeper in scale.