I always enjoyed racing on most of GT5/6's fantasy tracks, but I am worried that, if brought back to GTS/7 (whatever they call it), they might get the GTS 'treatment', and be ruined by the liberal slathering of huge rumbles and runoff areas that can be massively exploited. If there's any real difference between GT6 and GTS, I'd say it was this. The whole philosophy of HAVING to stay on track boundaries has changed for the worse, IMO.
Many of GT6's fantasy tracks would be ruined by the amount of legal shortcutting that GTS allows.
Mind you, given how badly you are allowed to abuse track limits even on the real tracks, I guess the OP is kind of moot. T1 at RBR shows just how bad it is.
I wonder what happened to PD's decision making on this? I remember several updates to GT5 and GT6 that considerably tightened track limits and did away with most of the egregious shortcuts. Why was this philosophy abandoned?
Bottom line, though - If you want real tracks, PC2 is already available. It's unlikely GT can catch up to their track count in 10 years, at this pace! But fantasy tracks, particularly the GT5/6 ones... That's where GT's edge came from. Great to race on (especially road cars, GT6's focus), great to look at, infinitely playable, hard to cheat in the GT6 era. More, more more! And enough with the track width runoff exploits!