I'm not getting the "Burnout-clone" claims, to be honest. For me, the open world of Burnout Paradise added very little to the Burnout "experience" and, actually, if anything took the focus away a bit. I suppose a bit like the change between NFSU and NFSU2 - the open world in both cases were relatively uninteresting and simply got in the way of progress (much as a driveable "world map" would in GT). Burnout was always about the silly, fast-paced Ridge Racer "drift" physics and gratuitous crashes, not exploration / free driving, for me. But to each their own.
On that note, I feel the game that best approaches my attitudes to what a free roam mode could be like, in terms of just driving, is actually GTAIV. Combined with the best driving physics of the series and the excellent location and its interactivity, I had more fun just moseying about Liberty City in a Blista (or whatever other mundanity I happened to brutally seize at any particular moment) than I did doing the actual storyline.
Interestingly, TDU2 did nothing for me, (way too dudebro in attitude for a start), its driving was dynamically numb and the actual road layout was a touch everyday / uninteresting / seemingly not made with driving in mind (they were after all trying to make a believable world, which is mostly full of dull roads).
I'm not suggesting that GT should allow us to get out of the car and assault fellow drivers, just the same as no one is saying GT should copy Burnout's driving model.
I know we're all relative beings, and so we can only really compare to what's been before, but don't stymie the discussion by simply stating "we don't need an x clone". That's just lazy, short-sighted and unimaginative - which is probably only par for the course in this age of spoon-feeding...
Why don't we try to imagine how an open driving mode could work in the context of a Gran Turismo game, and actually bolster the entire package, rather than dismiss the idea entirely based on unrelated games?
As has already been mentioned, there are plenty of people who just run practice sessions simply to drive the cars. It would be nice to have a bit more variety and free choice in those instances, just to go where you feel like it.
What I think people really want is an interconnected network of driving roads, not necessarily a "city" like Burnout, GTA, NFSU2 etc. As a start, think the Special Stages all linked up and connecting to more rural roads etc., not boring grid-based cities. Now think about all the best driving roads you've ever been on, all interconnected with next-to-no "filler". A lofty goal, but surely worth the time and effort.