I just thought of an idea that would be perfect on the DS. Phantasy Star Online with it's WIFI connection.
Think about it, you can move about with the dpad, Action (attack, talk, pick up item, examine, etc) with A, critical attack with B, and preset magic with the other buttons and Start opens the menu. The touch screen on the bottom can have a little keyboard on it so you can touch the keys with the stylus to talk to people online.
I'm expecting my check from Sega for my idea riiiiight....... NOW......