Better (fractional!) scaling on HiDPI screens for applications that don't do it themselves (Java: I am looking at you!)

energy consumption. On Linux is worse than on windows.

Need an "editor" for the fstab, thats the only thing I still have to use the shell.

Random color picking at install.
So I don't have to do it manually to discriminate all the systems I ssh into.

an app that stops me from distro hopping 🤭

problem: language barrier and accessibility barrier

Free worldwide internet phone calls to regular phone numbers.

idiot proof setup of samba shares so I stop getting tripped up with user permissions.

An app (with a beautiful adwaita-UI) with tips how to improve the users system, agnostic to their skill level and detected configuration.

my vote goes to a text-analyzing and processing recipe-based tool like #cyberchef (which is great, btw) that guides the user through the tasks to perform and iteratively builds the appropriate code or (sed|awk|RE|cut|etc) commands to simplify or transform the input whether it starts as delimited or free text or randomish garbage, cTLDs to handle when building pihole blocklists, filenames to fix, directory names to clean or simplify, whatever.