DRM 1977-1981 with both Division 1 and 2.
or
IMSA GTP 1984-1991 with GTU.
Add light RPG elements like F1 2018 (i.e. car development) and turn it into a car lab (like Motorsports Manager) where your car might end up looking (and performing) quite differently than what happened historically: What if the BMW M1 Turbo had been properly funded and developed? Should Jaguar go with a V12 or Turbo V6? Should we research front diffusers before the AAR Eagle gets them perfected?
This would take it out of the realm of just another dry driving sim and turn it into a game as well.
License the latest Madness as the game engine, but find a way to use Automobilista's physics and FFB (which Reiza has hopefully figured out already!).