After having become adjusted to the bicycle, I've found that power control via peddling is a very reliable and easy-to-use system. However, actually providing the power via said means, while very efficient, puts an upper limit on what one can do and for how long one can do it.
Seeing this, I periodically consider the idea of a vehicle which uses peddling to control the output of, say, an electric motor. Shifting could be accomplished via a scaled-up bicycle hub gear, to handle the increased torque, yet still keep shifting fairly simple. Alternatively, a CVT could be used with set "gears" or even a fully-continuous ratio selections without getting "stuck" in a gear. An electric motor would also prevent stalling if one selects too high a gear, and would not require to be idled when the vehicle is at rest. It would have to incorporate a bicycle-style freehub as well, since coasting is coasting regardless of the power source.
Using pedaling instead of a pressing down a pedal or pushing a button would additionally act as a sort of dead man's switch, which would make it safer. To allow one to shift down when in too high a gear, there would have to be some sort of mechanism to prevent motor damage.
The end result would be a vehicle as easy to operate as a bicycle, yet with increased speed and range. It would additionally remove what I perceive to be a disconnect in control associated with a throttle button or pedal, which is an issue I see with current electric bicycles.