Well my idea is to have the same city that GT5 had from its main menu which rarely appears at times, or it was just used there which I hope it wasn't. It should be complete since I have many ideas that's about to be shared. It can even be the all new GT Life mode with rules of the road and obeying the speed limit kind of style, being online/offline. It doesn't have to be a big city, but make sure theres many districs to drive too for hangouts.
Home Garage
Of course, every GT game must always have a home garage, no matter what. The home garage is you store your cars in with no limit added.
GT Air Lines
The airport called GT Air Lines (Gran Turismo Air Lines) can be the new phototravel, for players to travel the world to take photos of any (PREMIUM) car and avatar.
NCD (New Car Dealership) & UCD (Used Car Dealership)
The new (PREMIUM) car dealerships could be located everywhere in the city for each manufacturers. While the used (PREMIUM) car dealership is located at some parts of it or outside of it.
GT Auto
The new GT Auto is also located at some parts of the city, sometimes near gas stations or stand alone garage buildings, are based on GT5's GT Auto but better and with a new feature added. Livery Editor.
Tuning Shop
The tuning shops that are also located in some parts of the city. Where players can purchase various of upgrades and parts for their (PREMIUM) cars.
Racers and Drivers Tests (Licence Test)
In the main part in the city, the licence test building is the only place to head too in downtown. Where players can do a racing test called GT Racer's Tests, is a classic test based on GT but with leaderboards. This is the best way for he or she to beat their friends and everyone in the same region or around the world, to top fast times to be the leader on the leaderboards and master of the GT World.
A new feature called GT Driver's Test, is where theres no leaderboards, but being tought to master road rules and signs of the road in order to earn the GT Licence. This is the best way to become the best responsabile driver of the road in the visual world, and the real world.
If you want to race, the race event station whatever to call it. Its where you can join online/offline races or time trials. There you can also create your own races again online/offline, or just have track days (Free Run).
Well. That's pretty much I have so far, hope this will change your mind about thinking GT should or shouldn't have a free roam mode. All of those ideas came directly from my mind filled with imaginations.
Edit: I almost forgot to add touge roads outside of GT city for drift lovers, along with gas stations.