There does exist a high powered 1,000 watt geared rear hub that takes a modern Shimano 7 or 8 speed older 9 spline type cassette. I remember seeing a post about it on the endless-sphere forum. It is like one of only two rear hubs out there that take a cassette instead of a screw on freewheel. It is very expensive if I remember correctly. I'll do a quick search over there and see if I can find the thread.