Lever arms
Lever arms on cranksets can increase the power output (well) over 50%. One such system has been designed by Maurice Houbracken. The image on the right shows 2 systems inspired by Penny-farthing bicycles.
Elliptical sprocket wheels
A similar objective can be achieved by having sprocket wheels on chain drives which are elliptical, rather than round. For example, if the sprocket that is attached to the pedals of a bicycle is elliptical, with its major axis perpendicular to the axis of the pedal cranks, and with the axle of the pedals passing through the centre of the ellipse, then effectively the gear ratio of the drive varies as the pedals revolve. When the rider's foot is driving downward, the gear ratio is highest, to achieve highest speed. When the rider's feet are moving horizontally, with little force, the gear ratio is lowest, giving best mechanical advantage, and getting the feet past this alignment as fast as possible.