Wide support in macOS for sharesheet-style extensions, along with better built system supported extensions (why can’t I copy links this way in every app?)
Meaningful improvements to Mail.app. It’s still the most stable mail app, but it feels really stagnant.
A unified language for app design, such that the macOS native look and the iOS native look begin to converge. I care a lot about the usability standards on macOS, but the actual look of a standard “mac app” is aging. Also, there’s little precious about that compared to things like standard Menu items and shortcuts, how the title bars work with files, etc.
New entitlements, widely available for Mac App Store apps.
Improvements to performance monitoring, maybe even some options to allow macOS to kill apps more aggressively ala iOS that can be optionally turned on. I want to know more about what is crufting up my system and I want more incentive for apps to be better written.
New tiling options for window management rather than just full screen or full screen L/R.
Jason Becker