While something fresh is a good idea for GT, this iteration looks to have done away with car collecting and tuning in favor of 4 fixed classes. I find this a shame, since this activity was always a major differentiator and the change leaves that gameplay completely to Forza. It also doesn't match the photo mode portion of the game very well.
I was thinking through the DLC approach which GT Sport will need to take. In reality they can't charge for tracks without splitting the player base uncomfortably (although this has admittedly never stopped iRacing, but that's a different market). They can add new DLC cars as long as those cars are balanced to the classes (or potentially form new classes). The limit to 4 classes at launch does seem to be deliberate, though, so the question is how they will handle that.
In the end, I just expect that the audience for a focused multiplayer racing (and screenshotting) game is much smaller than the car collecting audience. Forza Horizon 3 will turn out to be by far the best selling racer of 2016 despite GT Sport being the first Gran Turismo release on PS4....