I mean, the X360 is far more powerful than the Wii, as far as hardware's concerned, so trying to make a game like Metroid wow people more than Halo 3, that's gonna take a lot more thought than Miyamoto knows how, at this point.
Well its not always about how powerful the hardware specs are, but how you make use of what hardware you have (which very many Wii games are lacking lately). The folks who did the first two Metroid Prime games did a good job on it visually and gameplay wise. They're expect to do the same if not better with the next Metroid.