Randomly stumbled upon this topic, and I find it quite stimulating.
An open world Gran Turismo (likely a spin-off?) game is a pretty thrilling perspective, but that would be hard to achieve, but definitely not impossible, though.
From a marketing/sales standpoint, that would be a proper opportunity for Sony to have a Forza Horizon-like exclusive on their consoles.
The thing is I highly doubt Gran Turismo's current engine could handle open world so well, so PD (or the studio developing the project) probably would have to switch to another engine; which would be a difficult trade as they probably would have to redo a lot of the physics, which would be a challenge on its own.
Engine-wise, I can easily see Sony providing a "race-ready" refurbished version of the Decima engine, but that'd definitely take some R&D time as well.
As for the game itself, it would have to stand out a great deal to become a best seller. If they manage to keep the car porn/enthusiasm to the current level, with the incredible handling, an interesting setting (why not Japan?), a nice and quirky car roster, the same "motorsports/road cars" balance (with actual race tracks and roads to race), that could be enough to make it a viable prospect.
RPG mechanics/storytelling could actually make it work and make it somehow unique - it's been ages since a racing game didn't have a proper storyline, but I'm convinced it's not a lost cause, it was done well before and could be even better now.
I've never discussed the subject with my coworkers at Codies who play GT, but I'd love to know their views on the matter