a gba cartridge memory card eh? I'd like if sports games had the new rosters on the gba cart, but sports games on portables haven't reached anything that great yet as far as the game's engine. I think a 2nd generation sports game could make huge strides as far as the game engine and gameplay. After that though, the gba roster would be cool, but can't the DS access online? Meaning you could just download the new roster online? Anyways, I don't see any of the mentioned roster ideas actually happening for sports games.
I think it would be cool if there was a DS game enhancer cartridge (in the DS slot), that allowed you to play your old gba games (in the gba slot), with new features exclusive to the DS, enabled by the DS game enhancer cartridge. It could sell for the price of a DS game or a bit lower, and be compatible with like 5 games. Or they could release them for certain franchises, like Advance Wars, Golden Sun, Mario Advance series, Castlevania, etc. It would be much like the super gameboy and gameboy advance player. One idea of the top of my head would be to let you use the stylus to play Advance Wars 1+2, and have other DS additions, etc.