I suppose somebody could develop a game that does not require the analog stick/nunchuk, and instead uses two Wiimotes. If that were a four player game you would need eight Wiimotes on one console (though, as stated, I don't think this is technically possible if the Wii conforms to bluetooth standards).
Outside of the current obvious peripherals such as the balance board, a keyboard and mouse, and a headset, it's possible that a future redesign of the DS will add bluetooth functionality as well. The Wii could have been built with that possible expansion path in mind.
I fully expect to someday see multiplayer Wii games and channels that utilize the DS as their only input, such as a trivia channel similar to what you see in every sports bar and wing joint in the US, where the DS Lite replaces the old NTN "Playmaker."
