How big a nfs:underground type spin-off for Gran Turismo could be.

It worked for Forza because Horizon outsold the main series (on Xbox one).

Gran Turismo is an iconic brand that sells 5x more than it's competition (which is incredible and makes it the Mario of the racing genre). Sony needs to make a new Polyphony team dedicated to it because they clearly can make focused products as GTS proves but fall short in comparision to other 'collect-a ton cars' racing series.

5-10 million sales potential. Sony please hurry up, this brand should not be left to die. Other publishers would kill for a franchise that sells 80% of it's copies on the name alone. 

"Sony please hurry up, this brand should not be left to die."

Wow, super dramatic lol.

Isn't Mario the Mario of the racing genre?

It is when you really think about the last sentence of my post. 

No, Mario is the Dark Souls of the racing genre.

For fuck's sake, know better.

Yeah, a spin-off (Mario Kart) selling better than the main platformer games validates my point.

I would buy an open world version of GT based on a Japanese map. Yet not if it puts any strain on PD, they need to be making more tracks for GT Sport!

GT World or GT Street. I've always wanted a TDU or The Crew sized map with GT's driving physics. I doubt Kazunori Yamauchi would be interested in that, striving for perfection in driving simulation doesn't really gel with an arcade like setting. He'll probably want a true modeling of Tokyo which might be ready for release for the ps6.

Nah, I don't like the idea of spin-offs. GT Sport should have been GT 7 imo. The main series sells so well that it really don't "need" to go down the road of spin-offs.

Nah, F:Horizon and The Crew is already something like that. Tokyo Drift type of game would be something fresh and nostalgic at the same time.