Regarding the poll in the OP - I do not think PD have "maxed out" PS4. Not when we have examples of Need for Speed 2015 and Driveclub as demonstrations of what this hardware is capable of. PD may have reached the level of visual fidelity that still allows to keep consistent 60fps (which is what the series' target has been ever since GT3), and even then that's debatable
I do think that racing in the morning, evening and night looks incredible (PD just handled the lighting really well), somehow it's daytime racing that looks...kinda plain? Something just makes the game look very pastgen during those conditions.
But regardless, since PS5 is going to be much more powerful than PS4, mostly on the CPU side, I would like PD to finally start upping the polygon count on track environments, proper antialiasing (it's very poor in GTS), improved shadows (still can't solve this one since GT5, fourth time's the charm), reflections (again, mostly daytime, but here we'll have the help of native rayracing support - GT would look jawdropping with RTX ON), and PLEASE FOR THE LOVE OF GOD get rid of jpeg-quality static 2D skyboxes, it's embarassing to look at