As good as some of the old tracks were I think GT need to move forward instead of re-adding old tracks. Adding more and more real tracks with weather and time change is much more important to me than re-adding made up tracks from the past games, in my opinion.
That being said. One of my favourite tracks in all of the series history is Citta Di Aria. Those narrow streets and gradient changes made for a very fun lap, if not a long one. Re-adding that wouldn't go a miss for me personally, especially if they added time change to it. racing round those narrow streets under headlights would be awesome.
But, I'd sooner see them add something new like a Ronda based version of Citta Di Aria that has a longer lap.