KERS Kinetic energy recovery systems will be the future. Pugeuot have one for years that spins an enclosed dense metal disc very fast like 40,000rpm or something when you brake or go downhill,that is connected with a clutch to the drive train when you step on the gas for instant punch out of the corners.Also there are many F1 teams that have an electrical version of this developed and ready to adapt to road cars when the demand requires it allowing small eco engines to be used.