......Ooooh an open world GT game discussion: love this topic.
Standard cars can stay, used by AI only, as traffic. Midnight Club LA did this, Test Drive Unlimited did this, so why not open world GT. Hell, pretty sure other open world racers do the same (The Crew, FH). As for premium race cars, why not keep them - allowing you to utilize them in "closed road-course" races, as well as all those tracks that needs to stay in game too.
So, basically, you drive around, find landmarks like car dealerships, places of interests, garages, gas stations, tuning shops and entrances to race tracks, all strewn across a varied map. To the west, you get Venice Beach-esque scenery, complete with American dealerships, races and challenges for muscle cars and the like.
To the north, you get "posh" mountainous areas simulating The Olde World (i.e. Europe) and your European dealers/tracks; and when you enter "CBD" area it's all tall skyscrapers and Japanese makers/tracks.
To the east you get a desert with lots of room for drag/off road, while to the south, you get some epic cruising routes flanked by the ocean and a cliffside.
The game needs to have a fast-travel system - if you found a landmark, then just press a button and boom, you are there, kinda like The Crew. And some sort of messaging system, where your friends can send you PMs and invites to a lobby; also for AIs regularly challenging you to a race, like in Midnight Club.
Also, this being a GT, not GTA, you've gotta obey the traffic law - the way I envisioned it, the game has a discreet EXP system where every action you perform nets you some points. If you misbehave, your EXPs are deducted. And with these points you can periodically claim bonus goodies such as new livery (yes!), new tuning parts and such.
And the physics needs to be GT, not GTA too. Add to that day/night cycle, full-on weather/season system (snows in winter, dry in autumn etc), some sort of damage system, and more accurate tyre wear/ fuel usage/ chassis and motor degradation, then we got ourselves pretty much perfect template of an open world racer. 👍👍