Hot take: I think some of those older original courses are overrated, and they'd need remarkable changes - much like a real circuit would undergo - to better facilitate online racing. I think Apricot Hill, however, is fine, and potentially Autumn Ring. (Meanwhile, Midfield seems a bit dull.) But classics like High Speed Ring, Trial Mountain, Deep Forest, Special Stage, and Grand Valley seem like they'd really need some serious updates. I mean, AFAIK, these courses were made for up to six cars in the PS1/PS2 era, not sixteen or otherwise larger fields we've seen since the PS3 era. I don't even think they really offer that much in scenery nor actual driving, at least when compared to some of the other GT-original courses (including city circuits) that've come since.
I can see that hard right after that first tunnel at Trial Mountain being a real PitA in online races, with those rocks that are all around it, as well as the hill you can launch yourself off of that's just before the start/finish line.
The best way I could see the revival of those three courses in particular, is by limiting the amount of cars that can be on-track to the same six as the original, and still making some changes for some corners, like that Trial Mountain turn. Adding more run-off areas where the geography would permit it - stuff like that. Otherwise I don't think it'll be fun, and that it'd mostly be the nostalgia talking.
EDIT: I had a few more points.
-A sizable amount of circuits from prior GT games seem like they'd be much better as point-to-point time attack stages, like Matterhorn from GT6, or the many circuits that only permitted two cars at a time in GT4, both tarmac and dirt. I think they're mostly terrible for racing.
-PD may wanna take a look at how other games tackle original circuit designs set in real locales, like the NFS Shift series, the GRiD games (including the 2019 title), and the Project CARS series. I think that could help direction a lot. For one idea, I think they could potentially scrap Tokyo Expressway, but bring back both the Special Stage tracks (including R11, and also have some of them interconnected into new layouts) AND the real-world R246, and THEN also model more of the actual Tokyo highway layouts which could also connect to R246, like the Wangan line and so on.