If not a brand new game built from the ground up, why not take RR6, remaster it and add every car from RR7, 3D and Vita (including special machines from RR PSP - 1 and 2), and make the car list perform equally in every category (which, what I meant, every car from the previous car class carries onto the next, up to Class 4).
As for tracks, we just need those from RR7, mixed with the PSP games as well as 3D and Vita. Oh and not to mention the cutscenes for when you unlock the cars.