I think it's a terrible idea. With the resources that would have to put in to model such a "world", 5 or more closed-course tracks could be modeled. It takes PD a year to model a racing circuit for GT5, it would take far longer to make an entire city or region of countryside. With the amount of detail PD demand of themselves, I just don;t think it would be worth the effort. There's a whole world of permanent racing circuits (licenses required) and closed-street circuits (whether something new like Guia or Montjuic or bringing back an oldie like Opera or Seattle, no licensing required) out there to be modeled by PD. They could put resources into developing the track creator or they could create new environments for the track creator. They could bring old fantasy tracks (SSR11, Red Rock, Apricot Hill etc.) back with GT5 graphics and weather effects. They could upgrade existing GT5 tracks to a higher standard and implement dynamic weather on those that don't have it. There is so much PD could do to improve the game, another new feature is not needed right now.
And if you have a realistic open world, it would essentially be street racing. There would be no driving safely, following the speed limit or avoiding accidents. That's not GT.
GT has always been closed-course circuit driving and racing and with GT5, the core of the game still needs to be enhanced with more cars and more tracks (with weather). I just don't think a complicated intensive large new feature like a free roam mode is what GT5 needs when the core values are not at an optimal level. I think GT needs to triple the number of tracks, implement weather on all non-oval tracks and triple the number of premium cars to get to a level where they could start looking a large new features that aren't related to core GT values.
