This is not going to happen whatsoever. The only chance of this happening is maybe if you are able to construct your own Autocross track. I never done Autocross, but the only chance of having the ability to create your own track is likely in Autocross. Autocross normally involves a certain number of cones scattered across a course. Each cone tells you what to do. For example, one colored cone would tell you to turn left, another color would tell you to turn right, while another series of cones means a slalom around the cones. To me, this is the only chance you'll be able to create your own track. I don't anticipate on seeing you have the ability to make your own track.
The only other possibility of track creation is if PD held a contest to people to create a totally fantasy course, either based on a real city (Seattle, Tokyo, Rome, Grindelwald, etc) or a completely fictional permanent or temporary road course. Maybe even an oval too. And then let's say PD is looking for three or four great designs. The winners have their tracks featured in Gran Turismo 5, much like the Best in Show winners at the Gran Turismo awards got their cars in GT4. Other than that, building a track takes a great deal of perfection, establishing braking points, pit road entry... it's a lot of work, and I know from being a forum participator on the PC game "Sports Car GT." It is not that easy no matter how hard you try.
The only other game to feature a track editing feature was Moto Racer 2. No matter how you tried to build a track, you had limits. A track had to be a certain left, have proper geometry in the course design, and the most number of laps you can do was 6. Then when you made a track, you could choose from five pre-determined scenery arrangements. So no course looks completely distinctive from another. There are two ways new tracks can be used: (1) downloadable add-on packages, and (2) if GT was available on PC.
Long story short: no.