Carbon fiber might even be possible before metal, for at least the structural frame. Probably wouldn't pass a single NHTSA test, but imagine how cool it would be if you could assemble your vehicle this way, with a CAD system doing the heavy math calculations, then go to a junk yard and get your suspension, axles, motors, etc. Plug the motor into an rPi that is your (whatever they call automotive computers, ECU?). Not sure how you'd do the interior or exterior, and the SRS would never work right. But you could theoretically get most of the way there, to a dune buggy.

downloading a car is easy. Getting it registered at the DMV is the hard part. Very easy attack vector unless this is just going to be a street racing car.