The idea of an adapter is really exciting to me, but it clearly is not a sensible solution for portable systems. It's a little hard to imagine a multi-system portable that isn't going to look insane. I thought the Atari-&-NES handheld Ben Heck made is almost elegant, but even then you have a cart sticking out at a ninety degree angle.
At this point, I think my interest in future portables would revolve around systems that haven't been touched on yet, such as the Atari 2600 or PC Engine/TurboGrafx-16. People have been practically begging on AtariAge forever for a portable Atari, and I personally would love to have a portable HuCard player.