I'm expecting the Wii/DS capabilities, but nothing huge. They've done it for the last 3 generations in some form but kept it pretty minimal:
Super Game Boy Player - SNES
Transfer Pack that came with Pokemon Stadium - N64
GBA Cable that was only compatible with about 3 games - NGC
I expect we'll see something around the 3rd or 4th year that'll be pretty cool and new but not much after that again. I think they'll only go big with the wii/ds connectivity in 3-5 games tops. And i expect all of those games to be Nintendo published. I wouldnt expect anything exciting from third parties using this. As someone noted above maybe just as an extra controller method or bonus for a second player but nothing really jaw dropping.
EDIT: I also forgot about the Gameboy player for the cube. just add that to the list above.
