Tires and Exhausts
I'd like to see non-compressed air tires. Tires are always in danger of going flat, when that is not a good reason to stop driving. We should build tires filled with a biodegradeable elastic substance that will function partially as a shock, and improve shocks for a finer ride. They can be retreaded if necessary around a centerpieced sufficient tire. No one wants a flat tire.
I would also like to see wider exhausts for petrocarbon fuelled vehicles. It seems that a 'custom' exhaust can improve horsepower by a considerable level. These things are just shaped in the factory. Install larger exhausts standard to provide more powerful cars.