Okay, this would be a DS game with single card download play and online multi-player.
I can't really think of much of a way to make it worth buying/playing for single player mode, but the Multi-Player modes could be interesting.
This is kinda an expansion of the
Mario vs. Bowser game idea.
Picture you and a friend playing against each other in classic Nintendo battles!
First example would be the Mario vs. Bowser battle of Super Mario World. Mario Runs around, stunning MechaKoopas and throwing them back at Bower. Bowser has to try and get Mario - duh, but has a few other issues. For example, if Bowser doesn't throw a MechaKoopa out of his Clown-Ship in time, the MechaKoopa-making-machine inside will explode - this ending the game for the player playing Bowser. Also, occasionally while modifying the Clown-Ship for a different mode of attack, Peach will use the distraction to her advantage and escape long enough to throw power-ups to Mario - and if Bowser doesn't quickly put her back underwraps, she'll just keep throwing Power-Ups to Mario!
Now, let's see a classic Donkey Kong style game, one person playing Mario attempting to save Pauline and the second playing DK, throwing barrels down the construction site. Now, DK's pretty slow, so he can't just throw endless barrels down the ramps, but he can choose when to toss 'em down and, with the touch screen, pick the path they'll take. And what about a Donkey Kong Jr. game in this style?
Perhaps a Legend of Zelda battle between Link and Ganon, set in the Sacred Realm of Link to the Past - where the Ganon-Player gets to try to kick Link's butt with his various moves?
Any thoughts? Games you'd like to add?