Sorry for the delay in reply, it's been a busy week so far. Anyway, depending on how far you want to go into being inspired by the FIA GT Championship: I would make all of the distances 500 km with a race limit of 3 hours long per the 3 hour rule. I would also wait for GT6 so you can use Silverstone. It seems the best season to simulate would be 2001 as it used Silverstone, Monza, Nürburgring, and Spa for a 24 hour race.
I would keep the schedule at 10 exactly (not including the Spa 24 Hours and it would look something like this.)
1. Autodromo Nazionale Monza - 87 Laps - Monza 500km
2. Filler - 500 km
3. Filler - 500 km
4. Silverstone GP Circuit - 98 Laps - Silverstone 500km
5. Filler - 500 km
6. Filler - 500 km
7. Circuit de Spa-Francorchamps - 24 Hours - Spa 24 Hours
8. Filler - 500 km
9. Nürburgring GP/F - 110 Laps - Nürburgring 500km
10. Filler - 500 km
11. Filler - 500 km
For the fillers I would recommend keeping it as European as possible. The tracks I would use would be:
Côte d'Azur (Top choice, European)
Circuito de Madrid (Top choice, fictional, but European.)
Rome (A little tight, fictional, but European.)
Grand Valley Speedway (Fictional and seems like it would be American, but it has great flow.)
Trial Mountain (Same review as Grand Valley, but perhaps Asian instead.)
Road Course - Twin Ring Motegi (Asian, but great flow.)
Circuit de la Sarthe (not a fan of the flow or the size of the track in comparison to the others but it's a major European track, even though FIA GT Series never raced there. If we had the Bugatti circuit I would use that instead. Then again, Circuit de la Sarthe is in GTR2 - FIA GT Racing Game.)
If you don't think organizing a 24 hour race is possible then I would use Spa as another 500 km and remove a race from the schedule.
Recommend Car List:
GT Class
Lister Storm V12 Race Car 99
Dodge Viper GTS-R Team Oreca Race Car 00
Dodge Viper GTS-R Team Oreca Race Car 00
Ferrari 599 '06
Lamborghini NOMAD Diablo GT-1 (JGTC) '00
N-GT Class
RUF RGT '00
Ferrari 430 Scuderia '07
Category 2
Renault Sport Clio Renault Sport Trophy V6 24V Race Car '00
Gillet Vertigo Race Car '04
Honda ARTA NSX (JGTC) '00
Honda Castrol MUGEN NSX (JGTC) '00
Honda LOCTITE MUGEN NSX (JGTC) '01
Honda Mobil 1 NSX (JGTC) '01
Honda NSX-R Prototype LM Race Car
Honda RAYBRIG NSX (JGTC) '00
Category 2 or 3
RUF CTR2 '96
Category 3
Lotus Elise 111R RM '04
The italicized cars are a bit iffy. Those are the closest I could come up with for the Porsche's and Ferrari's. The Renault is extremely iffy. All of the NSX's are just because of different vinyls for the most part. I'm not really sure how differently they handle from each other. I wouldn't allow the '06 versions of the Super GT ones though. It seems like Category 2 is a bit faster than it should be, due to the lack of good N-GT Class cars you might just want to use the Category 2 cars as your N-GT class. As far as Category 3 is concerned, the Lotus is kind of slow. You might not even want to have a category 3 if it can't match up with anything. I selected the 111R RM '04 instead of the Elise '00 because it seems like the 111R RM '04 is the fastest out of all of the Elise's. If you're wondering about my schedule reasoning or my car choices. I've been getting a lot of my information from this
source.