The world needs a new source of energy, an unspillable source.

Ways of improve fuel economy in existing cars? Hybrid/electric conversion?

Ok so here’s the deal… I’m trying to come up with a business plan for a class for a company that would be able to increase MPG in existing, used, on the road, cars…
My first idea is to try and find a method of converting a gasoline powered car to a hybrid electric/gasoline… Would it be any where near cost effective? How long before we’d have the technology to do so?
How about ways of converting cars to run on electric power or interchangeable battery packs or things like that? I know its possible, but would any normal not-environmentally concerned person ever do it?
I want to come up with an amazing business plan that will blow people’s minds… I think a hybrid conversion would be amazing but is there any chance I could make it happen? Even if it is not yet entirely cost effective, is it feasible? Would it be helped with economies of scale i.e. the more conversions are done the cheaper they become?