oo oo oo... how about, like, a 'ghost' mode that you see in other racing games... like client side recording of the movements of the player, and somehow exporting it to other people and somehow getting the ghost to be nonsolid and semi transparent etc etc
i dont think this will count as 'realistic', so umm nevermind...but angel mapper did do a map where there was a special actor that tracked the movements of the players and they show up as a sprite on the other side of the map, so somehow just make it so that the movements are saved and replayable on demand...
have you dug deep into the workings of the svehicle system? like how the vehicles are coded to skid at certain times etc...