There really isn't any room for improvement IMO. They are already the best in the business by a huge margin. They should focus on improving the visual quality of the tracks.
You aren't looking hard enough. The LOD is awful outside of Scapes mode. Especially with custom liveries. Go do a time trial real quick, save the replay and try to take a race-picture within it. We are talking PS3 quality. It's awfully low resolution.
PS5 GT will hopefully look like Scapes mode or close to it. the PS4 can only render 3 cars in Scapes mode. Imagine a full grid of them.