I think this all the better reason why GTS (and likely future titles) are taking a quality over quantity approach to the car lists. That said, I think you could still have a diverse group of vehicles that are balanced for each other, with ideally as few cars (that aren't meant for one-makes, like the Red Bull cars and the various F1 cars) going into Gr.X as possible.
On this topic, I wouldn't mind if we had events that used more "sub-rules," like how this week's N300 race specifically limits you to a group of JDM cars from the 90s and early 00s. I could see an N100 race that specifically limits you to older compact models like the Fiat 500L and the Volkswagen Beetle 1600, for example. I'd love this idea even more if we saw the additions of real old Japanese classics with under 100HP like the Honda S500 and the Subaru 360.
To be frank, I'd almost go far enough to say that almost all Daily Races should have more specific car selections, with the exception of Gr.X one-make races, Gr.4, and Gr.3 events. It'd offer a much wider diversity of events, but also could have players ultimately using a wider variety of cars, rather than just allowing any car within a given category (albeit with BoP and disabled tuning), making it a battle royale of sorts where the meta-dominating car appears a lot. You could have Gr.2 events where you can either only use the newer GT500 cars, or the older ones from the late 00s, for example. Or you could have an N500 race where you can only use the made-for-game road cars.