Would this be something that can be properly simulated with this physics engine of Kunos?
Surfaces:
Point to point races (hill-climb, rally..) are possible. There is a time control system for it.
Physics for gravel (if you want to make a rally stage) is "particular". You can make materials and change grip, sound, bumpiness, but not be able to simulate wheel "dig" or abrasiveness as in RBR. Bumpiness is a simple wave function (to be used ONLY on grass and gravel traps). The track surface uses a highly detailed physical model so no need for bumpiness. But of course it all depends from the artist.