I might be all be myself on this and may get rocks hurled at me but I'm thinking all new design. I guess I'm talking moreso related to chassis design.
My reasoning for this is that unless it's based on a different chassis, it's the same old, same old. The Challenger, while being a great car, very fast, but performacewise usually is in 3rd place when it comes to keeping up Mustang and Camaro. If the exterior were to be a rehash of an old design, I think some people would think that the car is based on the LX or whatever platform the current 300 is based on, which the Challenger is based on, which doesn't lend itself to be competative enough.
So, although I would love to see a retro '71 Cuda appear, I say all new design, exterior and under the skin.