Paper Mario DS (a new game, not a remake), another new 2D Mario Platformer (Super Mario World 3? New Super Mario Bros. 2?), new 2D Kirby game w/ full 2-player download play, Kirby's Dream Course 2 w/ full 4 player download-play, Elite Beat Agents 2 (please!), etc, etc.
Generally speaking, I want to see more games that feature full 2 or more player download-play, using the same approach as Mario Party DS (which works great.) No more mini-game only download-play. If that isn't technically possible, then sell multipacks which include one full copy of the game that can be played standalone, along with 1 or more "multi-card play" cartridges that can't be played standalone, but can be used for full multi-card play when connected to the full copy. I.e. they contain the full game, but are neutered such that they can't be played stand-alone. Obviously for a much lower price than buying multiple full copies of the game.