Well, USB and PCI on the C64 ain't going to happen anyways, so why bother.
I saw USB adaptor for MSX-1. As it fits the joystick port, I can't see any reason why it wouldn't work on C64 aswell.
Too bad it was compatible only with devices, that need only power through USB, such as lamps, fans and fridges :-D
(so basically it just connects power line to matching line on USB, no control over anything really)