On the weight front, Lotus has done a very interesting study:
Green Car Congress: Lotus Engineering Study Concludes Vehicle Mass Improvement of 38% by 2020 vs a Conventional Current Vehicle Can Be Achieved at Only 3% Cost
I've been working on a list of things that all car makers could be doing to improve efficiency:
More ways for auto makers to build more efficient vehicles - Updated 13 Apr 2010! - Vox
One of the things to add to what Lotus has done, is to add multiple latches to the doors -- this would improve the the strength of the overall structure, allowing the weight to be reduced because the door opening frames do not need to be reinforced as much. And the overall shell is complete, which increases the strength.
Another thing that adds efficiency in several ways, would be to redesign the wheels and the suspension, so that the wheels are rigid and non-inflating, and the suspension does all the dampening -- and the shock absorbers could then generate more electricity than they otherwise would. This would eliminate the need for an alternator; especially if, you had a plug in charger.
So, greatly improved rolling resistance and reduced unsprung weight, which leads to reduced energy to keep the car moving, combined with regenerative shock absorbers (which can be tunable for ride and handling), which eliminates the need for an alternator.
I hope you can read the rest of my list!