Well, the SD card adapter WAS a good idea on paper, but in reality... egh.
It'd be a great peripheral idea if it came out at launch, but if Nintendo's planning on releasing it- well- it takes at least a year (usually two) to develop a game. If they were to release it tomorrow, there wouldn't be many companies outside of Nintendo that could REALLY take advantage of it. Don't get me wrong, I could see where a GC Pokemon game that works with the ECard peripheral could really benefit from such a device (scan Pokemon, get them in the game later even if Nintendo hasn't invented them yet, battle it out with them at a friend's house).
Still, by the time it would get any meaningful support, Nintendo will have moved on to their next system. You figure it out.