For the sake of confusion, I hate using "X" to mean "cross." You know... how people use AutoX, MotoX (or just MX), SuperX (or just SX), and stuff like that. Now on to my proper points.
Autocross involves driving skill as opposed to all-out speed. This is the kind of racing in which a Toyota MR2 or a Pontiac Fiero can potentially beat a Ferrari Enzo. This is the ONLY circumstance I can realistically see making your own track. I think I've only played "Tokyo Xtreme Racer Drift 2" as far as any autocross-style track. That game had two types of Gymkhana courses- one for time trials and one for drifting. There are nine big cones laid out around the course to determine where exactly you go. This would be a simple model for this. A more advanced model would allow you to make purely competitive track.
However, I do have SOME gaming experience with autocross. It was back in the Sports Car GT days. One track was a course built around a fictional Hilton Hotel while another was some course with Denver as the setting. Even back dealing with some of the punks I had to deal with (including a Moderator) at the old Speedsims forum, one person challenged me to an Autocross race. And when I said no because I'm not an online gamer, he thought I was a coward. Of course, I'm happy to be on GTPlanet for 4.5 years since. Done right, this can really be a hit, especially online.
Let me show off as much as I know about Autocross. You have a bunch of orange cones to outline the outer edges of the track. There are some different-colored cones to denote left and right turns. Taking out a cone results in a time penalty. The race isn't over if you take out a cone, just that you get extra seconds added to your time. You of course want the quickest time, so hitting cones and cutting corners is not the way you want to go. Polyphony Digital's creativity will mean if we see PD use GT tracks and utilize them for various types of courses. It's like what I mentioned in the past about proper drift courses in GT for example. Another fun thing about Autocross is that you can actually have racing events held in major metropolitan areas so that more people can enjoy hometown racing. While not full-blown, FIA-credited race tracks, you can still enjoy racing in a mostly safe environment.
While I sound like an embassador for autocross, this is an idea I can be interested in as well as a great online gaming environment.