Gran Turismo 6 might turn into a full sequel on the PlayStation 4, according to Polyphony Digital CEO Kazunori Yamauchi. At a Gamescom panel, Yamauchi said that the (as yet not officially announced) PS4 version would take a while to arrive. By the time it hits, it could be such a different game that they'd simply call it Gran Turismo 7.
Joystiq reports that Yamauchi said he felt the "vision" of Gran Turismo 6 might not carry over into Gran Turismo 7. GT6 is coming December 6, and Polyphony is planning a year of updates, so the PS3 version is likely to demand their attention for a long while.