This will never happen, it just won't. Talent / Money / Legal-nes isn't there... but, oh well. I think it could work.
Nintendo DS Cartridge Slot-
Simple, a Nintendo DS cartridge. On the cartridge would be the Emulator itself.
Game Boy Advance Cartridge Slot-
Simple again, a Game Boy Advance Cartridge. On the cartridge this time would be the "rom / roms". The Emulator will load all "roms" from the GBA cart.
Nintendo DS: Upper Screen-
Simple, This would be the Screen where the Game Playing would be viewed.
Nintendo DS: Touch Screen-
The Touch screen will display the Loading Program. Which wou would chose the game you wished to play, adjust settings, etc... Durring Game Play, There could be a pause button (on screen to make use of the DS start button for actual game play) in which would allow you to pause the emulation and further adjust settings / save / load / exit a game.
IF Somehow, someone manages to program an emulator that took advantage of the Wi-Fi abbilities... this could also be a virtual Keyboard / message writing area (PictoChat). Something similar to what Kaillera offers... Though it will be a cold day in hell before someone manages to be able to do that.. let alone most of the functions I've mentioned.
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...I think I've gone way ahead of myself, as I know nothing about DS's capabilities, let alone anything about Emulation. I just think this would be the way to go if you were a Billionaire who had the resources and abbilities to create DS and GBA games and new his way arround emulation.