Just what it says on the tin - cars (or at least some of them) each get their own set of challenges (usually corresponding to its vehicle type), which would include:
-get a specific amount of race/derby wins
-get X drifting distance in Y seconds
-get X amount of airtime
-max upgrade the vehicle
-reach the max scrap amount
etc…
The prize for completing the challenges would be a unique visual customization item (Special car variants!) or just a lot of credits.
I think this would both encourage people to grind lower rank cars and (coupled with my special car variant suggestion) would add a reason to add more cool versions of the same car
revive, more potential playtime is always better
This idea is awesome, gives end-game users more to do cuz the game is boring as hell without stuff like this
revive
for all the peeps who want mastery back (including myself) 
revive
rvv
another revive
platina is not enough
this also gets a revive
revive this sounds right up my alley 
but honestly this is one of the best suggestions i’ve seen here
revive
heyyy maybe game suggestions can have tags as well