The original GT tracks are some of my favourites, so I really hope they stay in the game, with some upgrades of course.
I think
@MatskiMonk made a good point about the GT circuits being "designed" for road cars. They all have very nice flow. I find all the original tracks keep you active throughout the lap, ie, there are no super long straights that let you "catch your breath" like at Silverstone or Monza. If I were to make a comparison, I'd say all the old GT tracks are quite "Suzuka-esque" in their design.
I agree with
@MatskiMonk about converting Trial Mtn and Deep Forest into Tarmac special stages as well. I would much prefer this to having them converted to a closed circuit. I enjoy racing next to the cliff faces, and with only a guard rail between me and a 50' drop

There are many Tarmac special stages still in use today that, when compared to a modern closed circuit, are considered extremely unsafe (events like Rally Monte Carlo, Isle of Mann, along with many of the Hillclimb events held in Europe, come to mind). Everyone knows it's dangerous, but that's part of the excitement.
Along that same note, I'll never fully understand the obsession with making every track in the game "as real as possible". The virtual canvass is a blank one, capable of rendering almost anything the human imagination can think of - so why limit ourselves to
only recreating reality?
If you start looking around at most real world circuits, the majority of them are incredibly dull and boring in terms of the scenery around them. Complete generalization, but most modern circuits are built in the middle of a field. I would much rather have picturesque mountains, dense forests, or other visually stimulating scenery surrounding a track than a blank horizon.