What I'd like to see is a solution to play GB, SGB, and GBC cartridges on the DS. A slot-2 solution that, perhaps, would be designed to fit such cartridges underneath the DS, instead of sticking out from it.
Only thing is, I think I remember reading something about GB-type carts and GBA carts operating at different voltages, and that the DS doesn't provide the GB-type cart voltage in slot-2. So, don't know how probable that idea would be.