A Monza 500 would be interesting indeed.
How about a series of endurance events held over the year in which the total distance covered is used to determine the endurance champion. It will mean that if a car develops a fault, the team will have to fix it and put the car back out onto the track to continue clocking up the miles.
Maybe they could mandate driver changes, which will force the teams to use their reserve/test drivers in the races, as they will still have to run their two cars. Of course, there will have to be a mandated minimum time in which they are must refuel and change the driver, to avoid compromising safety.