Given that the full identification of a device is a VID:PID pair, and that's a 32-bit space, picking one at random is not very likely to result in a collision. Furthermore, this assumes true randomness and not "randomly pick one and then check online to see if some other device has already claimed it."

It also assumes that your project isn't close enough to some existing one that you really need to define a completely new interface instead of just using or extending that of an existing device.