Well I think it is possible to build it like a linear motor.
Linear motor - Wikipedia, the free encyclopedia
In the attachment I sketched what I am thinking of. Basically it is a permanent magnet AC motor with only a small portion of a stator. In fact I just inverted the one shown on wikipedia.
Thing is that you would need AC inverter and a controller for this. I am also unsure how to wire it all. But with this you could brake regeneratively and also the wheel wouldn't get that much heavier. With a battery the bike is heavy anyway.
Also, while heavy wheel might slow the acceleration, make a ride a bit bumpier and stabilize the bike(gyro effect).
