That's why I suggested some real GT4 cars in a previous post, atleast the more unique ones, like the Ginetta, Alpine, KTM, Panoz... Adding an M4 GT4 when the Gr.4 version is already in the game, or the Mustang, is kinda pointless, as both the real GT4 and fictional Gr.4 car have the same purpose and based on pretty much the same car. I'd rather see variety.
GT7 will have the Ford GT and R8 LMS Evo as new GT3 Cars. Don't remember if there is anything else, atleast so far. The problem with racing classes is that there are too many, and cars get updated super fast. When they release a game in one year, the year after the roster for that class will be outdated anyway for most cases. To be able to keep up, they would need to focus on those classes, which would hurt the rest of the car list. This is why they need to be selective with which cars they do prioritize.
The modeling resources on GT7 went to variety and tuning options. With only 80 something new cars for GT7 compared to GTS, it wasn't expected that they would update the entirity of the racing classes.