Some ideas for different race types:
Vehicle Specific -The person creating the race can specify a certain vehicle to be used in the race. Players will have to have that vehicle in order to race.
Vehicle Class Specific -The person creating the race can specify a certain class of vehicle to be used in the race. (Example; Bikes, Sports Cars, Trucks, Regular Cars, Special vehicles)
Pre defined start positions -Each race will have a pre defined starting grid and players joining the race will be randomly placed on the starting grid. Players not in the race wouldn't be able to enter the race area.
Joining races -Players could use a command (/race join) to join the currently active race and would automatically be placed into the proper vehicle for the race (if the race is vehicle specific). If the race is class specific the player will be shown a dialog allowing them to select which vehicle (from that class) they will use for the race. If the race isn't vehicle specific the player will be shown a dialog allowing them to select any vehicle they want (except forbidden ones).
Reset position -During a race it is common to spin out and go miles off the track. A /reset command or even a key press that will respawn the player at the last race checkpoint they passed so they can continue racing.
Easier way to make races -Have races stored in files so they can be easily edited or created.