I had an idea!!
Seeing as how the DS can access both software simulatniously, wouldent it be possible to release something along the lines of a Sonic & Knuckels?
Imagine buying Zelda Four Swords DS. You play thought it and have a blast with friends and pretty much get your use out of it. But then, when your finished, you can throw in you GBA LoZ: Minish Cap game and the DS card would eiter add new levels and items to the old GBA game, or perhaps even use the Minish Cap dungeons as a template to create new Four Sword multiplayer dungeons.
You perhaps you could go the other way around. Lets say that Jam With the Band NDS game becomes a big hit and nintendo wants to release expansions. Well instead of recreating a new game on a NDS cart, they could just throw new songs on a GBA cart that you use along side your NDS game.
You could also, theoretically, input an NDS card with the right softward to add touch responce to existing GBA games. Although I don't know the necessity for that seeing as how the GBA games were created with controll pads in mind...
Just some random thinking....