While it doesn't seem like there'll be any new classes, I still think it's nice that the PP system returns. This, when combined with car regulations, could still lead to some neat groups of cars being at least somewhat balanced with each other when PP limits are enforced. That said, I'm still holding out for a "Gr.1V" for various vintage prototypes, especially now that we're getting both the Alpine A210 and the return of the Chaparral 2J.
EDIT: And then there are still the four Le Mans racers from the 1950s, between the cars added to GT Sport and the ones that've won awards at Pebble Beach - namely the Cunningham C4-R, the Maserati A6GCS, the Aston Martin DB3S, and the Jaguar D-Type.
If not, then I at least hope for an expansion of Gr.B, either at launch or via DLC, as there are some automakers that were well-known for their actual Group B cars, yet are not in Gr.B at all, like the Lancia 037 and the Renault Sport 5 Turbo. (Never mind other famous Group B cars that weren't in GT Sport, like the other Audi Quattro S1 and the Peugeot 205 T16.)
I also seriously hope the brakes get fixed on the Vulcan, they're quite terrible in GT Sport. Honestly, I think that car is better-suited to Gr.X, alongside the Pagani Zonda R and McLaren P1 GTR. (Hopefully that's also where the Ferrari FXX-K goes, too.)
EDIT2: The return of the PP system honestly seems even better when you think of the N-Series cars that seem unusually better due to aerodynamics, or under-used due to weight, respectively such as the Toyota SF-R Racing Concept and the Dodge Charger SRT Hellcat. With the PP system returning, these cars could find themselves among much more appropriate competitors.