I'm starting to think PD chose Trial Mountain as the first classic original track to showcase because they felt it would be one of the most affected OG track in terms of changes, so it would come as an even more shocking reveal, positively or negatively. I also put down a little theory trying to guess how much the other popular classic tracks will change based on their layouts and general scenery, using a scale from 0 (less likely to change) to 10 (more likely to change):
Apricot Hill Raceway - 0: The layout of this track is perfect and fits a real life circuit very well.
High Speed Ring - 1: A good example of an old-styled high speed circuit. It definitely won't need a drastic change.
Mid-Field Raceway - 2: Despite being an 8-figure track with 2 tunnels, it still has a pretty realistic shape. Some additional run-off areas should be enough.
Grand Valley Speedway - 4: Here I could feel some little issues with the slow section of the track, especially with those few turns before and after the first tunnel. Maybe that would force PD to stretch the track a little bit, not like the twice-as-large Trial Mountain stretching.
Deep Forest Raceway - 4: A very similar issue to Grand Valley could be the case here. I can see PD removing a corner on that quick right-left-right-left part after the second tunnel in favor of a wider run-off area.
Trial Mountain Circuit - 7: I listed a track we have already seen here just so you can see how I would rank this track among the others.
Autumn Ring - 9: In my humble opinion, if Autumn Ring is really coming back, get ready to see even more radical changes than those in Trial Mountain. No, it is not because of the spiral bridge, it's because the track is so "compressed" together, there is a lot going on in such a tiny space. I feel PD would really change various aspects of this track, starting by drastically increasing the overall length more than a kilometer, just like they did with Trial Mountain. This would include increasing the main straight and some straights between the corners.
Honorable unlikely mentions:
Red Rock Valley Speedway - 1: Well, I feel kinda sad PD has completely forgotten about this precious gem. Its layout is surprisingly fun and realistic, plus it would come as a perfect shot for GT7, since the track would need almost no changes and would hit the nostalgia factor so hard.
Tahiti Road - 3: Another track I used to love. Combine a very simple and beginner-friendly layout with a breathtaking scenery and you would get another very smart addition for GT7. In this case, as this track is not that popular, PD could spice it up a little bit by modifying some sections and I'm pretty sure everyone would be okay with that.
El Capitan - 5: This track is probably not returning because Trial Mountain "took its location". But I included it here because it was a very fun track to drive on and it would be another really smart addition for GT7. Maybe stretching some sections would increase the overtaking spots, without killing the extreme aspect of the course.
I didn't mention any city circuit because most of them are based on real roads. As for the Special Stage Routes, I feel that the Tokyo Expressway is - unfortunately - a definitive replacement for them all.