Either way GT7 is now the outlier with a desperately outdated Gr.1 line-up (I'm not factoring in Group C as this should be it's own historic class). GR010 aside, the most upto date prototypes in the game date from the 2016 season, added shortly after GT Sport's release. Same can be said for GT500 and the much discussed GT3 line-up which forms the basis of GTWS.
I feel like GT7 should take its time somewhat, even if I totally agree that there are numerous racers I could see coming to pretty much every group. Gr.1 is obvious enough, between the various LMH/LMDh cars that are currently running in IMSA/WEC, let alone other brands that've shown interest in potentially joining in the future. As for Group C cars being in its own class, I think instead, they should have their own tags, as should LMP1, LMH, etc. so that they can be included or excluded, without PD needing to make a separate BoP just for the Group C cars.
If I were at PD, I'd focus on updating the most egregiously old lineups, like for Hyundai and Ferrari. The former should be easy enough, as I could see a made-for-game Elantra N Gr.3 alongside the road car, plus maybe a TCR-spec machine for Gr.4. Ferrari could totally get the 296 GT3, too, though I'm unsure if what sort of Gr.4 car would be a good update. Meanwhile, Lexus could even get the GT300-spec LC 500h for a newer Gr.3 car, as it competed alongside the class-winning BRZ that we already have.
That said, for some brands, I don't know if anything can be done to update them, namely for brands like Mitsubishi and Dodge - though perhaps the latter could see a made-for-game Gr.3 car based on the upcoming Charger.