GNOME:

• support tags in all applications and have combined search for them (e.g. let me tag e-mails and files, and when I search for my tag the tagged emails and files show up) (AFAIK GNOME developers already said, this will never come, because it would confuse GNOME users. Apple and Apple users have this feature for years now.)
• Bring back F3 dual pane views in the file manager, having two windows side by side is not equivalent
• Integrate and polish dash to dock or dash to panel, I don't care which one just make it work perfectly OOTB.

Custom per-folder themes in Nemo with drag/drop templating like os/2 had. Extend to all apps, actually.

wlr-screencopy-unstable-v1

Or ext-screencopy-v1 if it gets merged

GNOME, turn off screen backlight dimming?

CardDAV in the built in contacts app without installing Evolution, just to configure functionality that is there without Evolution. People have been begging for years, and submitted unaccepted code, but Gnome devs are going to Gnome dev.