Gran Turismo 7 should be the next one in line, but we always forget that the Polyphony Digital studio usually brings a Prologues before a big launch. According to a report on eurogamer.net, Gran Turismo Sport is a bigger game than the usual Prologues, but it's still far from the full Gran Turismo experience.
It's interesting that Sony decided to be vague about, knowing full well that they are going to be asked right after the conference. Gran Turismo is the premier racing experience on PlayStation 4, and it's a platform exclusive.
From what Sony said until now, the upcoming racing game will focus a lot more on the online segment, which is only natural. The console experience is much more social, and the new Gran Turismo will probably focus on the multiplayer part.
With that in mind, we also need to remember that there is not launch date attached to any of those two projects.
In fact, it's quite possible that Gran Turismo 7 will most likely be launched in 2017 if history taught us anything. Polyphony Digital is not known for its short development cycles, so be prepared to wait, a lot.