Cadillac Sixteen - Previewed back in 2003, this car traced Cadillac's roots back to the Cadillac V16 of the 1930's. It had 13.6L V16, the largest engine ever made by a car company, producing over 1000hp. It also featued a unique engine deactivation system, that used only four cylinders during regular driving, eight cylinders during passing, and all sixteen when you put your foot down. This not only led to decent fuel economy, but I imagine an incredible feeling during high speeds. The interior was covered in luxurious leather and wood upholstery, as well as a steering wheel made entirely of crystal. This is by far the Cadillac that definitely sets the standard of the world.
Jeep Hurricane - Revealed back in 2005, the Hurricane takes many things from the original CJ's, including no doors and no roof. There's also only room for two people. Underneath this carbon fiber body are two 5.7L Hemi V8's, each producing 335hp bring the power to a total of 670hp. This means a 0-60 mph time of 4.9 seconds, making this the fastest Jeep ever. It also has independent suspenion all around as well as independent wheels, which can turn inward and outwards all seperately. This allows for any type of manuevering including crabbing and going absolutely nowhere at 45 mph. By far, the most intense Jeep ever conceived.
Dodge Demon - Revealed back in 2007, this two seater roadster was designed to compete with the Pontiac Solstice and Mazda MX-5 Miata. Unfortuntely due to financial difficulties, this concept never came to be. If it had a bigger engine instead of a 4 cylinder engine, it would probably be an incredibly fun car to drive.
Jaguar C-X75 - If you ignore the fact that this is a hybrid, there is actually more to this car than meets the eye. Revealed back in 2010, this hybrid car will run on four electric motors, one for each wheel. Instead of charging the batteries using a regular gas engine, the batteries would be charged by two gas turbine engines, the same engines that power jet aircrafts. The inside of this car is not only luxurious, but incredibly futuristic. Last year, Jaguar decided not to build this car due to the state of the economy. The technology use in the C-X75 will be used in future Jaguar cars.
Ford Nucleon - Revealed back in 1958 as a small scale model, this was used as a demonstration for what nuclear powered cars may look like. This car would've been powered by a small nuclear reactor in the ack and would function the same way as a nuclear submarine. This car never made it to a full size concept, mainly because regular gas engines became the norm for cars and nuclear power became more expensive. I would love to see this car made just to see how fast nuclear power can make a car move. Of course, there is the possibility of over exposure to the uranium that powers the nuclear reactor and the feeling of driving in an atomic bomb that could go off if somone crashes into the car. Still would be a neat thing to see.