I had some ideas regarding the classes:
-Would there be issues if BoP also affected downforce & final gear ratio? Furthermore, would it be an issue if, more specifically, BoP in general had the goal of making each setting be as uniform as possible between cars? With each car in a class having identical power, weight, downforce, and final gear ratio? On one hand, it would make things potentially more balanced, but I could see this potentially stripping some cars of their relatively unique driving characteristics. But I think accounting for downforce and final gearing could potentially help a lot with Gr.1's case and the Group C racers.
-The N-series could come with the condition that tuning is not only disabled, but BoP is also disabled, overall resulting in the cars being as stock as possible. Instead, only very narrow selections of cars will be permissible at a time, mostly modeled on events from GT games' campaigns, like those events where you could only use the "Gentleman's Agreement" JDM cars, or perhaps if there were an event where players were limited to the various Kei cars.
-In addition, there could be a series of classes for tuners, which could include the S-FR Racing Concept, the Roadster Touring Car, various SEMA winners, and cars customized by the player, especially when they've installed additional parts both internally and externally, such as aero kits and rear wings. Tuning would typically be permitted to some ability. Sometimes all settings will be available, or maybe just a few specific sections. There also may be range limitations on a setting.
-In addition, there should be a way to make the field of cars as diverse as possible. For example, when the car is provided, the car you end up could also be random. This could force players to adapt, rather than having relatively homogenous fields of cars that follow the meta. Also, qualifying in daily races would be much more similar to that in championship races, wherein you get a limited amount of time to qualify, and you also must use the car you set your best time with in the actual race.
-There could be a group for drift cars, like the BR-Z, but also the D1GP-spec RX-7 FC that was picked from the Tokyo Auto Salon. These would have BoP applied insofar that the power & weight is identical per car.
-Another group could be for the newer "hypercar class," and some cars currently in Gr.1 could be moved there, namely the cars that don't resemble LMPs as much, like the Bugatti, Alpine, and Audi VGTs, whereas Gr.1 cars that do look a like more like LMPs like the Mazda and Hyundai VGTs would stay. But perhaps instead of making a new class, there could be a subset of Gr.1 classes. Like a Gr.1a for the hypercars (and similar VGTs like Audi, Dodge, Alpine, & Bugatti), Gr.1b for LMPs (and similar VGTs like Mazda & Hyundai), and Gr.1c for Group C cars. Or maybe they could really all work together, I really wouldn't know - but I think if they truly were to work as a single Gr.1, it could be a lot better if BoP accounted for downforce & final gearing, and possibly if BoP attempted to make all affected settings as uniform as possible, rather than adjusting BoP through time trials conducted at PD's end. I think allowing both PD and the players to view usage stats (ala the Pokemon Global Link) would help a lot, too.
-On the other side of things, I don't see there being enough cars in Gr.B, Gr.2, Gr.3, nor Gr.4 to warrant a similar set of sub-classes being created, but that's just at the moment.