What if companies made unlockables based on having certain GBA games.
Imagine having Mario Pinball Land in the GBA socket unlocked a multiplayer pinball game in Super Mario 64 DS?
There could be a lot of possiblities with Pokemon games and GBA/DS communication. Transfering GBA Pokemon to DS games for example.
I don't think Nintendo will do this (at least not right away) because of all the slack they got about the connectivity with the GBA and GCN, but other companies might.
Final Fantasy I & II for the GBA could communicate with Final Fantasy III for the DS. You could upgrade stats or trade items and equipment between games, or even use characters from one game in the other.
Even if these types of abilities are not in the releases of the games, they could be added through online downloads using the DS like the way Pokemon can be added to the DS at Pokemon movies, and demos can be downloaded at game stores.
Imagine the homebrew possiblities with the DS.
Someone could make a DS card like a Game Shark or action Replay the had a GB/GBC emulator that would allow playing GB/GBC games in the GBA slot on the DS.
I have a lot more ideas, but this post is already long. What do you think?