there are plenty of emulators for the 3DS via a DS flash kit
tons of emulators for xbox, xbox 360 and PS3 aswell
There are tons of emulators on the Wii already, not just the built in SNES NES GameGear Genesis and more either, there are even a few DS converts on the Wii, as it happens 3 are shown on my last played list as those WiiWare games are just ports of the DS games
There are details missing from the 3DS that make it difficult to program a working emulation of the system, same applied to the xbox original (even though the original xbox is based on PC hardware, it is still not PC hardware) and applies to other systems even older that are yet to have a fully functional emulators
]
