Just go back to Forza Motorsport 4 and take every single car out of that game. Add newer cars for each brand and you got the best car selection ever in the history of Forza.
Actually it's an Australian based game therefore the game would be 100 times better with cars made in Australia such as, VY,VR,VS,HJ,HG,HX,LX SLR and so on. Also a King Of The Mountain expansion would be wonderful.