The things that date the design the most are the things that ruin the aerodynamics of these cars. The door windows is one of the big offenders. A retro style G body should use flush mounted door glass and framed windows. It should also do away with the large amount of windshield trim and instead flush mount the front and back windows as well. Then the headlights should be flush, and I would argue that the nose is too high off the ground and the base of the bumper needs to be lower to the ground. Remember that an important component of modern design is minimizing airflow under the car. Likewise, the front end would need to be smoother and lack protuberances such as the bumper guards, and anything else that sticks out or in. I would also forgo the formal roof line of the basic G body, and instead use the one from the MCSS Aerocoupe or GP 2+2. The door handles are also a major aerodynamic flaw and I would use a flush mount design instead. While I was at it, I would move the antenna from the front fender to the rear of the car. The gas tank would also need to be moved under the back seat and a fuel filler door added to one of the quarter panels to improve rear impact crash safety. This also helps with handling and weight distribution by reducing the polar moment aft of the rear wheels and the tendency of the fuel weight to act as a pendulum and generate a counter force to the direction the car is turning. Then there are the front fenders and wheel placement. The engine's crank pulley needs to be behind the front axle center line so as to help with the weight distribution, which would necessitate a longer section of metal between the front of the door and the rear of the wheel arch. Another thing on modern cars is the wheels. New cars do not have deep dish wheels for aerodynamics as the shallower the dish, the less the disturbance to air flow the wheel presents. Oh, and the door mirrors should be incorporated into the doors so that they act as one piece, and the drip rails would have to go. Finally, the windshield glass would need to be raked back far more than it normally is. I hope this gives you a few ideas!