you want a 50 button with thumbsticks etc.etc. gamepad take one, gut it, hook up wires to the switch pads and make a wiring setup to make it compatible to the minimig.
that would be way better than trying to make a usb interface.
for some of the other stuff like mass memory an ide interface would be easier as well and would work with simple adapters to many modern mass memory cards
there are schematics and software for ide stuff online
