My prediction is that some cars from the PS2 era that didn't make it to the PS3 era will also not make it to GT7, such as the Toyota MTRC, the Nike One, the Ford Model T, or the Nissan 350Z Gran Turismo 4 Edition. That's sort of too bad, especially regarding the former two - I really found it neat how in the Special Toyota Demo, the MTRC would have a dynamic ride height, raising it when moving from tarmac to gravel on the Grand Canyon course that was in the demo. I don't believe the final version of the Grand Canyon in GT4 had mixed surfaces like that.
At any rate, I am hoping/predicting that all PS3 and PS4-era courses return, as well as all premium vehicles - sans some concept models that could be replaced with production-spec ones. For example, I think we could replace the BMW 1 Concept tii with the BMW 1M Coupe. I also don't think there's a place for the Toyota FT-86 II nor the Nissan GT-R concepts from 2001 and 2005. I would, however, say the Toyota FT-1 Concept could stay, if only because of its connection to the production-spec Supra A90 and both the Gr.X and Gr.3 versions of the concept. I'd also remove the Toyota S-FR Racing Concept, unless Toyota itself can provide a cockpit for it - I'd like all cars in GT7 to have a possible cockpit view.
This is also before we consider some vehicles that were standard models in both GT5 and GT6, but had notable improvements between the two, as if they were on their way to becoming premium models, such as the Nissan R390 Road Car.
It would also be ideal if we could have the F1500T-A replaced with a formal return of the Lotus 98T, as well as the return of other F1 cars like the Ferrari F2007 and F10, as well as the more recent Mercedes-AMG W08. I think if we don't see the two Ferraris, it'll at least be more likely we'll see the W08 return.