I don't know how optimized the GBA emulator is, but I did hear it is a modified version of Visual Boy Advance. VBA takes a fair amount of PC power to run full speed in the more powerful games (at least, it did in single core PC's). But since it's running on DS, some shortcuts may have been possible. Since it did hold all the original hardware used for GBA games, and the processor is still an ARM one.
As for N64, I don't like to count my chickens before they hatch, but I doubt it will be done. I guess it MIGHT be possible, at least at slow-ish speeds (maybe less that 10fps). But then again, maybe not. Hard to judge what the iPlayer can do currently. All we have is a GBA emulator. And we don't even know how optimized it is. Many games like Golden Sun 2 aren't really very playable and go very slowly. I'm not sure if that's a lack of power, or a lack of optimization on the coder's fault.
Now, older systems are totally possible and easy to imagine. I could see superior SNES, Genesis, and NeoGeo emulators. Those are already fair to great on their own. SNES emulation on an iplayer might even be able to handle some special chip games like Mario RPG or Yoshi's Island. Maybe not, but I don't see why not.
As for N64 controls, I could see them mapping some controls to the touch screen. The C buttons for instance. L is rarely used, so they could map the Z button to L. Or just make custom configurations, maybe for each game. Whatever, it won't happen. Even if it COULD, I doubt many people are interested in iPlayer development. GBA was more of a novelty designed so you could play GBA games without slot 2.