Any PC gamer with a decent rig will extol the virtues of downsampling if asked about it. And with how high the resolution can be pushed with PS4 Pro, the image quality improvements for 1080p users should be pretty amazing in most cases.
It's the easiest thing for devs to implement as well. They have to effectively scale back this stuff to reach an acceptable frame rate. It shouldn't be a difficult feat to patch in for most games.
Titanfall 2
Producer Drew McCoy explained in an interview that the PS4 Pro version of Titanfall 2 will not run in native 4K. However, the console's extra power should help the game stick closer to its 60FPS target. Read the full interview here to learn more.
The Elder Scrolls Online
PS4 Pro with a 1080p display:
Increased graphical fidelity.
Middle-earth: Shadow of Mordor
PS4 Pro with a 1080p display:
Super sampling anti-aliasing.
NBA 2K17
PS4 Pro with a 1080p display:
2K confirmed to GameSpot that it will still render a 4K resolution even on a 1080p display. The company said this should result in "much better anti-aliasing" with HDR support.
Paragon
PS4 Pro with a 1080p display:
Enhanced visual effects, procedural ground cover, higher scene complexity, greater texture fidelity, and dynamic reflections.
The Witness
PS4 Pro with a 1080p display:
1080p resolution instead of 900p and increased antialiasing quality from 2x to 4x MSAA, while keeping a solid 60 FPS.
Killing Floor 2
PS4 Pro with a 1080p display:
Increased framerate and supersampling, as well as increased texture resolutions (which may also be present at 4K).
.....then stacks of other games where the details haven't been confirmed yet.