Not trying to say it should be made anytime soon, but how hard would it be to make ? and i know it might seem like it has no uses, but there are some games that use usb controllers as few as those are (like motion sensor controller)

There was some work done recently doing some preliminary work, but it's way too early for anything... USB support probably isn't nearly as easy to implement as you think it is, and it would take people away who are still working on the core emulation features which are still incomplete or inaccurate.

If someday someone create USB plugin would it be able to recognize any device attached or would you have to create it for specific device. For example could Gran Turismo 4 recognize Logitech Driving Force Pro wheel without any specific coding for that wheel?