Each player starts with a D-class car, and modifies said car to the top of D-class for road racing. A point is awarded to the player who wins the road race. Then, the SAME CAR will be remodified to the top of D-class for rally racing. Again, a point will be awarded to the player who wins the race. Next, the car will be modified for C-class road racing, with the same points system occurring, then C-class rally, then B-class road, B-class rally and so on, all the way up to S1-class road and rally.
Some vehicles should not be allowed to participate e.g. Peels, Isettas, possibly the Abarth 595 as well, but other than that the only rule is that the car must be in D-class stock, and be able to reach S1-class. Engine swaps, drivetrain swaps and extensive modifications are all allowed and probably necessary to attain S1-class. The winner is the player with the most points at the end of all the challenges.
In the event of a tie, the tied players will build their cars to the maximum potential possible with no class limits for road racing, and tackle the Goliath.
The tracks used for the races when I did this challenge with my friends were the Horizon Festival Sprint for road racing and Tarn Hows Scramble for rally racing.
I hope you decide to use this idea in a video, and if anyone else would like to try this challenge please feel free to give feedback.