I think the grid size should depend on the track, for one thing. And I think it will, as there will likely only be so many pit garages per track. But when there's a track where the maximum grid size is also the largest for any course, I'd personally love 30 or more, as I recall the real-world Nurburgring 24h can have hundreds of cars on-course at once. And this is hopefully in addition to multi-class racing. Heck, when the FIAGTCs return in GT7, there could be two Manufacturers Series: a Gr.4 championship, and a Gr.3 championship, with each round for each championship being essentially shared. Maybe there could even be a Gr.1 championship running alongside it, albeit with less automakers than the Gr.4/Gr.3 series. Same thing with Gr.2, especially if they add modern DTM cars, which would allow BMW, Audi, Mercedes-Benz, and Aston Martin into Gr.2 - as well as Toyota, with the new GR Supra GT500.
Although maybe not every class would be in every round. Maybe each class would only be racing every other, or every few rounds, but overall combined with another class each round insofar that every class gets an equal number of rounds. So for example, the Gr.1 Manufacturers Series would usually be racing at courses that you'd also see in the WEC, plus some of the original courses, and if there's a round at Le Mans, Interlagos, Spa-Francorchamps, and/or Nurburgring GP, it'd likely feature both Gr.1 and Gr.3, not entirely unlike the actual WEC where there's LMP1 and GTE at those respective courses.
With all this in mind, I also worry that PD would focus too much on having real-world prizes, and for the sake of costs involving said prizes, they'd only have two FIAGTCs as we've seen already. Still, multi-class racing could be real fun. Perhaps even the N-Series could get involved in the Manufacturers Series. I mean, I think with better restrictions on vehicles and tuning, more N-Series races in the FIAGTCs (Nations Cup or Manufacturers Series) could be very interesting indeed, such as a round where you need to pick a classic JDM roadster, and you'd be able to pick from the Toyota Sports 800, the Nissan Fairlady Z 2000, and the Honda S800. Or imagine a race just for the various sporty pickups, like the Toyota Tundra TRD Pro and the Ford F-150 SVT Raptor - especially if they add the Dodge RAM Rebel and Chevrolet Colorado ZR2.
I suppose I digress, but my main point I'd like for everyone to take away, is that the FIAGTCs have tons of potential for further development, and some of that could very well involve multi-class racing, especially with a potentially larger grid size in GT7. If there was a Manufacturers Series round at the Nurburging Nordschleife, for example, that could have potential to be an ideal multi-class race involving Gr.3 and Gr.4. But I really would like it if more classes were involved in the Manufacturers Series, as well as if the Nations Cup had more interesting restrictions on eligible cars per round. I'd at least be content if more automakers joined the Manufacturers Series, but I just hate it when we have a Nations Cup round that permits any car within a featured class, and then it just turns into a one-make with the most meta-relevant car. I'd much rather have stuff like some of those Daily Races where you could only use the Group C racers within Gr.1.