Off the top of my head, I'd like to be able to drag or minimize a parent window when it has a modal dialog open. This is especially frustrating when using an application that uses nested modals and you need to reference the grandparent window to fill in some information on the current modal.
They should buy the code for the file search utility named "Everything" and build that into the OS.
Also, the Amiga had multi-character drive names and path mappings/assignments in 1985. It was nice to type DOCS: or GAMES: into a file dialog to go directly to where I organized those resources. Sure, I could do this with single-letter drives but that requires me to remember what each letter maps to.
Perhaps much better interface for multi desktop and other multitasking.
Windows is so behind, and every step they take in the right direction is an authoritative foot-gun.
Most of its problems are, at their heart, symptoms of Microsoft's proprietary closed-source design style.
Apple made huge strides in the right direction by basing OS X on BSD, then proceeded to make every step from then on in the wrong one. Walled gardens are pretty, not functional.
What legitimately useful features can we expect out of the release of iOS five major versions down the line? Will there be any new features that change the status quo, and that we'd be practically unable to live without? The significant features like the app store and multitasking have already been done long ago. As for macOS, it seems that the new security controls and notifications are significantly impacting some people's user experience, but they're still "improvements" from Apple's standpoint.
And yet Apple and Microsoft are still somehow compelled to keep releasing new versions of their operating systems on a fairly regular basis. It's hard to imagine there will be a "final" or even long-term-support version of iOS. But those companies aren't going to market their new OS versions as changing nothing, so something has to change.
I'm honestly very curious to see what will happen ten years from now when the vendors will struggle to meet their own arbitrary standards for innovation with each new OS version when, by nature, there are not many more significant breakthroughs left to be had.