NASCAR, IndyCar, WTCC would be great to see represented in GTS proper, same with JGTC
F1 and/or F2 would also be welcome additions, but since Codemasters have the official iicense and the 2017 Mercedes is literally the only Formula car besides the Dallaras - it looks we might not see those ever, or not until 2021 anyway when the series undergoes through major regulation changes
I think just the W08 would be fine, as would a single, more recent Indycar chassis. I also think that as far as NASCAR goes, just one car from each of the major automakers would work, and maybe for each season that's been in GT, as well, rather than all the cars that only have different sponsors/drivers. At the least, I think the Jeff Gordon cars should be there since he was present for a segment in GT5.
But some older F1 cars would be good, too - especially those from prior GT games, like the two Ferraris from GT5, the Lotus that got replaced by the F1500T-A, and maybe even the actual F1 cars that the GT3 and GT4 formula cars were based on. In general, these formula cars would each be in Gr.X, as they'd be good for one-makes instead of trying to have notably different technology pitted against each other. Maybe to replace one of the GT3 F1 cars would be an actual McLaren-Honda car driven by Senna, as I'm pretty sure one of those cars from GT3 was modeled on one driven by Senna.
I think just having the real-world equivalent of the F688/S and F094/S would be enough, as that'd be two different engines (V6 and V10) and they'd be different enough as far as their years go, (1988 and 1994) versus having all six GT3 F1 cars added as their real-world versions. As for the F094/S, it's apparently based on a Williams F1 car that uses a Renault engine, so maybe it could be found under the Renault Sport brand - much like how the McLaren F1 GTR Longtail was obtainable at the BMW dealership in GT4, as it used a BMW engine.
In total, the F1 cars in GT, from prior installments, would be as follows:
Lotus 97T (aka F1500T-A)
McLaren MP4/4 (aka F688/S)
Renault Sport-Williams FW16 (aka F094/S)
Renault Sport R24 (aka Formula Gran Turismo)
Ferrari F2007
Ferrari F10
Mercedes-AMG W08
Of course, I'd love other F1 cars, too, like the much older Honda F1 cars from the 60s, or that Lamborghini-powered Minardi that was a flop in real life. I honestly wouldn't even mind it if there was a Gr.F for formula cars, as I think unusual fields of cars is part of GT's strength as both a game and as something to watch. And in that case, we could see a lot of manufacturer representation between the chassis and engine-builders. But I'd be fine if each car was in Gr.X and meant for one-make races, too.