With how long winded PD supports their modern GT games, I wouldn't anticipate GT7 to arrive until late 2018 at the earliest. The game arriving in 2017 doesn't work with the way that PD makes games (Slowly). For GT7 to release in 2017 it would more than likely be announced around this time next year, giving GT Sport around 6 months of limelight as the major game which doesn't work too well for what it's going for.
For those of you planning on skipping GT Sport and waiting on GT7, good luck, because it will take some time.