Super capacitors are a good touch
Most energy from regenerative breaking lands up getting thrown away otherwise: batteries simply can't take the charge as fast as a vehicle under braking generates it, so horrendous proportions of the energy are lost. A combination of batteries and super capacitors is better yet: the capacitors will be full well before the vehicle finishes breaking; they can offload that slowly to a battery so as to have room for the next time the vehicle breaks (say vehicle goes 60mph -> 40, coast, 40-20, coast, 20->0. obviously if you accelerate hard in between the advantage is negated.) Exactly this fuel cell \ capacitor \ battery malarkey is used in a random research project from Birmingham uni: http://www.birmingham.ac.uk/alumni/news/items/2012/10/Oct-University-builds-the-countrys-first-hydrogen-powered-train.aspx
Don't get me wrong, a car with a top speed of 60 and no luggage space is as much use to me as an umbrella on a submarine, but still I like the idea.