I understand that it's entirely too late to change anything with WWDC 2020 right round the corner, but here's what I'm hoping for and wanting to see:
Improved PencilKit APIs with shapes/stencils, color-picker wheel, more brushes, etc. Also, bring PencilKit to macOS. My parallax art app is the only one of my apps that's not also available on macOS via Catalyst, so this would be nice.
Improved CoreAudio APIs. Also, why are there so many audio APIs? Just unify them all into something that works across the board.
A system-wide color-picker on iOS. Mac apps already have this, and now with the Graphics & Design category on the iOS App Store, it would make sense to open this up there too.
Staggered revenue cuts for developers based on App Store earnings. Something like 10% for those earning less than x per month, 20% for the level above, and 30% for everyone else.
In-App Purchase drop-in modal API similar to the current Mail implementation, which will help unify the IAP purchase experience across the board and help create trust.
More AppKit elements via Catalyst. In its current state, Catalyst apps still look like iOS apps for the most part.
Media control keyboard shortcuts for the iPad.
The ability to remap keys on the iPad.
Low power mode for the iPad (and maybe for MacBooks too).
Display iPhone-sized apps on the iPad in the correct orientation.
Spotlight on iPad more like Spotlight on Mac.
Stock calculator and weather apps on iPad. Although, this is never going to happen.
Siri Shortcuts that happen silently in the background, instantly, without going through all the steps visually. If something has to open a web link as part of its steps, do it in the background.
Web inspector for Safari on iPad.
Real-time translation and subtitling on videos (and corresponding APIs).
Unobtrusive alerts and call screen.
Wireless screen sharing from iOS to macOS, or even between iOS devices. I would love to be able to share my screen to quickly show something to someone who's not physically with me or to a bigger screen.
Async iMessage picture sending that doesn't block the main thread.
Default apps (at least for mail and web browsing).
Changeable lock screen buttons (I've never used the flashlight lock screen button).
Improved HomeKit integrations. Let's get robot vacuum cleaners on there too.
Xcode on iPad. I know that this has been repeated time and time again, but it would be nice. Although, I'd be happy for an iPad-specific IDE that's totally different to Xcode on the Mac too. Just let us create apps on the iPad.
A native Pro graphic design app for the iPad. I want Sketch on the iPad, but an Apple solution would be even better. Along the same vein, Final Cut Pro on the iPad would be nice too.
SwiftUI improvements. I'm aware that many developers have expressed interest in a CollectionView API of sorts, and a variety fo missing features that would be nice. I haven't played around with SwiftUI yet to have a strong opinion on this.
Improved developer documentation (and more of it).
I think a lot of these are things that have been said by many other developers too, further enforcing the need for some of them. Either way, WWDC should be fun!