I have an idea after reading about rewind :
Have the cars like in real life, wrecked it once, you lose it, and will have to buy another one. Non terminal damages can be fixed for a price. If realism is the main goal, that would fit in.
This applies to all game modes, offline and online, free run and race - except Arcade mode. Have a rep system as well that linked to the racers A spec and license level, crashing to others / causing accidents often, and penalty will be given, suspension from races x days online, but can still do online free run.
Nobody will ram others for fun anymore, because the rammers will suffer as well. This will also make racers behave like in real life.
Once all that in place, give the rewind button at a cost of quarter of the cars price each time, do it 4 times, is like buying another car. The cost only applies to offline races, free run and online does not. Rewind is disabled for online race.