HDR and wide color gamut! While the displays are still only really available in the mid to high end (I don't count HDR400), it's no longer just pro gear and I upgraded to a new display recently that I'd love to take advantage of it with. I've been using the new, still in testing Variable Refresh Rate on GNOME and this would be the final piece of the puzzle for making me ditch windows 100% when it comes to gaming, as Proton has basically solved every other issue for me - I'm primarily a singleplayer gamer.
I don't know if KDE got it working yet, but Gamescope's works pretty well out-of-the-box. Nobara and maybe Chimara OS already have this ready with a session for Steam Big Picture mode.
Kinda funny that Windows games seem to always get compatibility with these things first. I guess just adding support in Wine means more games get the functionality at once than developers adding it on a per-game basis.