For smaller cards you can always put the USB connectors on a slot cover via cable connector. Some thing like...
http://www.shaxon.com/part_images/9/USBAA1-XKIT.jpg
Agreed. There are so many add-ons for the Amiga and the such that having something like this would make it more flexible and easier to fit in with other hardware.
I can see this working MUCH better with something like the Mediator which rises higher in my A4000T. If I wanted to use a card like this, I'd have to buy a riser and would most likely have a port or to covered because of the offset.
Getting back to the 060 Accel. idea of the Original poster, I totally agree that a CPU and memory solution only would be quicker, easier and therefore more likely to become reality. As other have mentioned, there are other solutions out there for what people are proposing as additions so keeping it simple would not only make it more likely to work, but be cheaper...