One cool thing iv'e seen is a USB mouse with 64MB of memory built in. That would be very handy to write a driver for. Of course you'd need some sort of port connector, as it uses a full size USB connector. +computer emulators would have to support it.
If you couldn't just use a USB hub, could maybe butcher a cheap unpowered one and just cross the wires over to give a direct link, it would just be acting like a bridge rather than a hub
It would be clumsy but it would get the job done, and would mean you could connect any full-size USB device that there was a driver for
Ive seen this portable hard drive out on the market that has a SMC (and other media) input, and it has a button so you can copy everything from the SMC to the hard drive. If there was a version of that that would let you copy back and forth, and had a built in menu for that, it would be great.
I had a microdrive at one time, and it was a pain in the ass - sooooo unreliable. I went through two and then gave up cos I kept losing photos (used in a diggital camera)...