I think the harnessing the turbo idea has some merit, but someone with some engine engineering experience could say for sure. I would also assume an average turbo would produce far too much energy for a normal cars electrical needs, if you have batteries for propulsion though...
I always think steam would be a good add on, a couple water filled tubes inside the exhaust after the cat to capture some of the waste heat, how to harness the steam is another problem again.
How about true wind power? Folks in the plains with a north south commute and a steady wind from the west could use land sailing rigs, they may not work so well in traffic though
http://en.wikipedia.org/wiki/Land_sailing
Ian