I search up about Choro Q HG2, also called Road Trip Adventure. It was a role playing game with cars only, but it was a large open world. It isn't a sim racer, but a fun one. Like @WedgeX thought up. Why not GT6's interface be a whole open world game all together. The main spot would be GT City, which is actually Special Stage Route City.

This is also what I thought up as well a few months ago, it was a nice idea.
My Home Garage, Race Events, GT-TV & News, License Test, and Commuity Museum where players from around the world can show off their cars and even have others download their designs from specific cars, will only be located at the city area. Except Tuning, Paint & Livery Shop, GT Auto with oil change, car wash, and many more. GT gas stations could be located in any area outside of GT City.
You can even find some rusted cars laying around too and restore them in GT Auto. Such as finding a rusted Ford Mustang Boss 302 '69 in a grassy off road dirt area near a wooden house, and accept to whether take it with you or not. If yes, it'll magically appear in your garage to fix it up in GT Auto later. These cars may also be rare and can be unlocked in arcade mode in GT City's Race Events.
The GT Airport for Photo Traveling is where it comes in handy. Only located in GT City. You may even find some abandoned airport areas to make hangout spots with your fun to be in as well.
I hope I didn't get carry away, but I have fun sharing my thoughts with others.
@diegorborges Thanks for that example, it was hepful and loved the scenery in that game.
@JBanton I think GT City should be the only city area. The rest should be towns and villages. Like your dock drag strip idea by the way. 👍 The harbour in the city and near a coast is an excellent idea. *.*