Interesting topic, I've heard a few lectures from high-end motor manufacturers so far on the matter, their opinion is becoming fairly clear after a decade of prototyping;
Most suggest all-elec is not the way forward except for city driving. Motors are massively more efficient than petrol engines and easier to control, but charging and discharging batteries isnt 100% effiient, and battery power/weight ratio is orders of magnitude smaller than that of petrol - it's simply not viable for anything other than very short range without a monumental leap in battery/storage technology.
KERS, or its electrical counterpart, is already in place in just about every hybrid out there - they regenerate on braking, supping the momentum of the car to charge the battery pack or next accel.
Expect to see hybrid everything eventually, making use of the torque from 0rpm that motors have for accel, but gathering the power from ever decreasing quantities of petrol/D through an engine.