Yes, yes. This is a good idea. This way we don't have to carry two devices if we want to switch between GBA and 3DS games. And maybe they can also make a way for people be able to download it on the eShop for free if they can provide proof of ownership.
Though I hope that if they do implement this, they could make it so that they could be read while playing another game, like how the DS can read the cartridge to add some extra features. But, even if they don't, being able to play is still a very good feature.