Why not? It would be an addition, not replacing tracks or racing.
Not sure about obeying speed limits though, although an online police-force with real police cars (and players being the police) earning credits for detecting other players speeding sounds fun to me, and obeying the speed limit might be worth credits or XP points too, etc. (why else obey them?

)
Taking away licenses might be too much though........
Imagine a city section (with surrounding highways like SSR7 too perhaps) like other games already offer but with real cars and GT's physics/graphics, etc. and playable online (okay a no-brainer frankly).
Sure it won't be for everyone but overtaking whilst dealing with oncoming traffic, traffic lights, etc. and experiencing the cars in GT in a new way aside from driving solely around tracks sound cool to me.
Petrol stations could be a starting point and ofcourse the place to refuel and change tires if necessary.
It would also be extremely suited as an online social meeting point, a cooler PS-Home for petrolheads (since the ability to walk around is mentioned), showing your car and perhaps go to a track to race or do other stuff in that city/town/industrial setting/whatever.
The possibilities are huge I reckon (for all I care add trucks and vans too to make deliveries, a new gameplay element, does anyone remember an old Japanese videogame called "Payload"?) and I'm openminded enough to applaud Kaz's thinking regarding this, if you don't like it don't use it I guess.
Speaking of Massive Open Online Racing, saw a bit of the 2011 Dakar footage recently and how cool would it be to have a large open terrain section without "invisible walls" but with natural borders to drive around and navigate?
Think
Red Dead Redemption landscape but instead of a horse, some 4WD horsepower (and more Dakar vehicles).
So what if other games already offer stuff like it? Other game offer cars and tracks too, just not in the way a GT-game does.
So yes Kaz please do, and when it finally does arrive in a GT-game take note how many of the nay-sayers will love it (as with anything that's implemented in a GT-game, very predictable.......).