I made an electric longboard a few years ago. Before they where popular, and you cant buy or legally use it in germany anyways.

It worked pretty well. I guess it could drive s.th. like 35 km/h for 10 km. It even had brake energy recouperation. A higher topspeed would be no problem, but its already very dangerous^^

It was a very simple design. Just a brushless outrunner attached with friction to one of the wheels.