Release a DLC for Gran Turismo 5 every two months if the feedback from the players is good.
The development of Gran Turismo 6 has already begun, according to Polyphony Digital unveiled itself in an interview with IGN the American scene.
Kazunori Yamauchi, producer of the series, has been quite clear: "Of course [the team] is working on Gran Turismo 6".
The justification for a new release is that GT5 "is not something that can be updated constantly with DLCs", so that those elements can come into this form will be included in a new game.
It also has advanced to launch a DLC for Gran Turismo 5 every two months with new packs of cars, but always "depending on the reaction of players to the first content already published"