Some of my suggestions for what Gr.4 and Gr.3 cars that could be added to GTS for manufacturers that aren't eligible for the Manufacturer Series portion of the FIA Gran Turismo Online Championship are:
ALPINE: The A110 GT4 Cup will be their Gr.4 car and the A110-50 could potentially be their Gr.3 car.
Agreed. It'd be really funny, too, since the A110-50 is a lot like the Renault RS01.
BUGATTI: They do have the Veyron Gr.4, which is one part of the eligibility done. Now, they just need a Veyron Gr.3 (or an EB110 GT1 if PD is somehow insane enough to insert 90s GT1 cars into the Gr.3 class, bar the prototype looking ones such as the Porsche 911 GT1 or the Toyota GT-ONE from 1998, the 1999 TS020 is updated to LMGTP regulations.)
IMO they should just make a Chiron Gr.3.
FIAT: The 124 Abarth Spider (assuming that the road car is coming into the game) is a good candidate for a Gr.4 and a Gr.3 version of the car. Think the hypothetical 124 Abarth Gr.4 as the replacement for the cut Mazda Roadster N200, which is planned to be in the Gr.4 class before getting the axe in favor of the Atenza Gr.4.
But that could just be under the Abarth label, and not Fiat. I'd also argue the Abarth 500 Assetto Corse could be good for Gr.4, too.
There may be more cars that Polyphony could use as a basis for their Gr.4 and Gr.3 cars, but that's it for now.
There's quite a few that could be added to Gr.1, too, and potentially Gr.2 when we get more clarification of how Gr.2 is defined - e.g. if it's really just Super GT500 cars. For Gr.1, there are Daytona Prototypes for Chevrolet and Cadillac, and older prototypes like the Lamborghini Countach QVX.
Ideally, I'd like to see the Manufacturer's Series have races from more classes than just Gr.4 and Gr.3 - maybe even the N-Series. (Though as far as BoP goes, I think I'd rather see each N-Series be balanced by
not including cars that can be in that N-Series via HP tuning or de-tuning. Especially if there are enough cars for multiple N-Series, which I think will be more likely when more cars are added in the next GT game.)
And meanwhile, there could be a larger emphasis on fun one-make races in the Nations Cup, especially for cars that have a connection to GT - this could be stuff like Pebble Beach or SEMA winners, the F1500T-A, the Mercedes-Benz W08, the various VGTs, the Nissan GT-R, the Amuse S2000 GT1 Turbo, etc. Or maybe each race in the Nations Cup could also serve as re-creations of classic GT campaign events, like an event where players can only use Mazda Roadsters, or another where only a certain drivetrain can be used. Though maybe that would be better for a non-FIA championship to have between the yearly seasons.