Firstly, I absolutely hate the shortcut bar on iPad Pro when using the Smart Keyboard - especially in Split View. I’d like to be able to hide/minimise it or invoke with a keyboard shortcut. It should also be black to blend with the bezels more. I’d disable it entirely if I didn’t need to AutoFill passwords with it.
Also on iPad, I want a way to switch focus in split view. Two of the same app side by side. And make it easier to navigate the UI using a keyboard if desired. Maybe some sort of tabbing system?
Dark mode based on ambient light/screen brightness as Maps already does. No brainer. People want it, works great on macOS just give it as an option and even better, have it just turn on when it’s the right moment (in a dark environment, or at night).
First-party app improvements - many have languished for too long without updates now. Mail, Calendar, etc. Time to give them some love. Safari’s address bar should be at the bottom. In general UI should be reworked to accomodate for larger devices and one handed use...
Refreshed iconography, sounds, etc. More depth, more mature. More how macOS adopted flat design... more detail in the icons, for example.
Ability to change or customise the lock-screen buttons (currently flashlight and camera) to other things like Mail, Messages, Home etc... with third-party API so devs can add their own options.
Reworked home screen - some sort of app drawer for lesser-used apps, widgets available along with apps (can take up 2x2 of grid, or 4x2, etc), dynamic icons (a la Calendar or Clock now).
DESKTOP CLASS SAFARI on iPad. Sick of seeing mobile/tablet websites on a 12.9 inch tablet. So done with it. Add Downloads manager to Safari for iOS, accessible in Files app across all devices.
Plug in a USB drive and access the files in Files app. Save from apps to USB drives if required. Improved Files app generally.
Password protect Hidden photos album, possibly also protect per-app in the app’s Settings. “Require Face ID to open Maps” for example.
Photos on iOS to get feature parity with Photos for Mac, especially with editing tools. Vignettes, clarity, etc.
Music. I want crossfade. I want in-playlist sorting (newest added on top, or sort playlist by artist). I want swipe on art to skip track. And something genuinely as good as Discover Weekly over time. Or I’m sticking with Spotify.
Snooze notifications for 5 min, 15 min, 30 min, 60 min etc. Sync notifications across devices. If I’ve actioned on iPhone, remove from iPad notification centre.
Better Do Not Disturb - options to hide all notification bubbles etc completely until it’s turned off so you can more easily detach from people while still using your device for web/music/etc. Recap what you missed when you turn it off again with a summary of all the notifications you got, grouped by app.
Step counter in Health - update it more frequently. No idea why it can’t be instant or near-instant instead of taking 10+ minutes to update the count.
End the stupidity of the overlayed volume HUD and the full-screen, un-dismissable/minimisable incoming call UI
Just a few ideas.