Polyphony Digital has indicated that development has begun working on the next Gran Turismo title, according to Eurogamer.
Apart from the vague announcement, not much else was indicated, including if the title will apear on both the PlayStation 4 and PlayStation 3 or if it'll be PS4 exclusive. And though he mentioned Gran Turismo 7 in the inteview, Yamauchi didn't specifically state that the Gran Turismo title under development at Polyphony Digital is in fact Gran Turismo 7, but he did say, "I don't think it'll be a Prologue," which means that the next title will follow the numerical release pattern.
Kazunori Yamauchi did state that the new Gran Turismo will feature cars that are standard to the franchise:
"I doubt that we'll be throwing away the standard cars. Each car has its own fans. So I think we'll hang on to the archive. In the meanwhile, some of those cars we may be able to make into Premium cars as they become available - but basically we're more focussed on increasing the number of premium cars."
On top of that, Yamauchi briefly mentioned that "the quality of the sounds in Gran Turismo 6 will be improved, but most of the work will be done in Gran Turismo 7." Perhaps the team at Polyphony Digital is working on new techniques to present game audio through Gran Turismo 6, but this is pure speculation until either Polyphony Digital or Kazunori Yamauchi confirm it.