I think any racing series would make a great game if it had a good career mode or online mode with great physics and also simulated the different teams and cars. Even something like gt3 should have differences between the teams running the same cars. As long as the series has variety in terms of cars and tracks it has the ingredients for a good sim.
That being said historic supergt would be amazing with both classes of cars. A late 90s or early 2000s year would be great. Supras, gt-r, nsx, mclarens, vipers, diablos, porsches, silvias, rx7s. Tap into that gran turismo nostalgia. Instant cash! Although I understand that the language barrier would be huge for any western developer. And while I don't really care about single seaters I think 90s era indycar game with good physics would be fun. Althought it suffers from small car selection even if back then there were multiple chassis and engine manufacturers. Not that many tho.
In the end I think touring cars from late 80s and early 90s would be the best option. Lots of international and national series to choose from, lots of cars and especially lots of tracks. The cars themselves are great and you have dtm, atcc, btcc and so forth to choose from. A lot of national series. But sadly I don't think people appreciate those cars enough for how much fun they are to drive and race. Hard game to sell.
I think any game needs to have more than one car or class of cars to be interesting. F1 games get away with it by having a good career mode and tons of tracks. But for a pc sim it at least should be moddable so people can add cars and tracks. A supergt/90s jgtc game quickly runs into the issue that they did not have many tracks in a season. Even in 2002 they had just 8 races and they ran two of the tracks twice so they only had 6 tracks that season. Too few for a racing game imho. Especially when the only tracks people know about are the suzuka and fuji (and TI aka Aida aka Okayama). A problem that can not be solved even with adding more seasons. That would mean more cars but the game needs more tracks.
And it is a hard series to expand locally with other series as most of those other series (say some of the national japanese and asian series) would be running those same tracks. If you expand to australia then of course you'd have access to new cars, tracks and series but in the end it would be just a bunch of unrelated series crammed into a single game at that point. What would you even call it at that point? Super japanese australian racing game? (please make it, I promise to buy at least two copies)