Honestly, I think the Lamborghini VGT would be a great fit for Gr.1, considering its general appearance and its power figure. I'd be nice for more manufacturers to have a Gr.1 car, between the VGTs, LMPs, Group C cars, and other machines. For example, DPIs could give a Gr.1 to Chevrolet and Cadillac. I'd especially like multiple Gr.1 options per automaker, just as we can already see this for Gr.4 and Gr.3. I could see Lamborghini getting two Gr.1 cars: the VGT, and the Group C-era Countach QVX.
That's before we get into how some automakers may only be able to get Gr.1 cars via older vehicles, such as the BMW V12 LMR, the Ferrari 333SP, or the Alfa Romeo SE048SP. But then there's the possibility of not just LMP1 cars but also LMP2, and that'd be good for giving Gr.1 cars for Alpine, Lotus, and MG. There's also the older GTP class, and the potentially upcoming LMDh class, which could have cars from McLaren and Lexus. And who knows where the late-90s GT1 cars could end up, like the Nissan R390 or the Mercedes-Benz CLK-GTR.
Just a lot of potential to expand the "staple" classes, to be honest. I think having an anachronistic or otherwise unusual combination of cars racing at once is part of what gives GT its identity beyond other racing games that strictly adhere to more realistic fields of cars. That said, I'd also like to see more events that restrict the cars, such as a Gr.1 race that only permits the Group C cars.
I'd also love to see - and this would likely be in GT7 rather than GTS - more classes other than Gr.3 and Gr4 as part of the Manufacturers Series, such as Gr.2, Gr.1, and Gr.B. Maybe even some N-Series classes! I don't really know what could go into Gr.2 other than Super GT cars, but I'd bet at least modern DTM cars could fit in, giving Gr.2 representation to Aston Martin, BMW, Audi, and Mercedes-Benz. Possibly even Opel, too.
This, of course, is also before we get into the possibility of new made-for-game racers, like the 4C Gr.3, or the Genesis Coupe Gr.B. It's getting me excited for GT7 just thinking about it!
EDIT: I wonder what automakers would absolutely need a made-for-game car? I could see Suzuki getting some, and then maybe there could be quite a few for Gr.2, like a more modern take on an Alfa Romeo DTM car. Then there are some gaps that could be filled, like a Gr.3 car for both Alpine and Bugatti. Perhaps a Chiron Gr.3? Though I think the GT1-class EB110 could fit into Gr.3, just as the '95 McLaren F1 GTR did. But an Alpine A110 Gr.3 car could look really cool, too!
Meanwhile, I don't see PD creating original Gr.1 cars, but I think tons of ground could be covered between DPi, LMP2, VGTs, the future LMDh class, and others like the GTP class or the older DPs, as well as other Group C and LMP racers. Maybe even WSC cars could be added as to give Shelby and Ferrari Gr.1 cars, as well.
This is also before we get into other automakers that have either been absent from GT for a while, or could have further introduction into GT, just as we saw the introduction of DeTomaso in GTS. In that regard, I wonder if we could see the return of Venturi? Or maybe a more proper introduction of Iso - perhaps even alongside a Gr.3 Zagato VGT!
