Ya know, with this Writer's Strike going on, don't doubt this Gran Turismo movie as something that could never happen. They're gobbling up game franchises all the time, so I'm sure they'd be looking for some kind of fodder for a big-budget movie.
That being said, what I'd suspect the movie could/should be like:
Due to the increasing lack of interest in the majority of motorsport, leaders of the major racing leagues decide to go "back-to-basics" and gather the best pro drivers of the world to create the new Gran Turismo Racing Association to supply new racing enjoyment. Drivers, to be selected from around the world, are all notable for their various talents and preferences for motorsport. Both veterans and "hot-shots" will be allowed to race, and clearly, a main point of the story will the the clashes of racing style and preferences for car types.
That, of course, leads into the main focus of the story. We'd have to select key members of the site to be the "main characters," although, certainly everyone would have to have some kind of screen time to be worth having in the picture. My guess is that the main story would have to be over the the sides of racing, presumably the new-school techies versus the old-school hot roders, versus the stereotypical Euro-rollers (clearly, the lines have been drawn for some members). Then, obviously, between the camps, we'd have to have some sort of story over the older folks teaching the kids new stuff, etc.
...So obviously we'd have to discuss who sits on what side and what kind of role they play in that group...
I think I'd be with the old-school hot roders (despite running a decidedly new-school Corvette), being one of the relatively skilled youngin's. I'm certain that I'd have a fairly friendly rivalry with McLaren (Euro roller?) and Azureman (techie Japanish?), but its hard to say.