I'd love something like a file system. Nothing as complex as the Finder, but more powerful than what we have now.
The App Store could use a lot of work. I would include that as part of the OS update.
I'd love to have an additional view option of my system in addition to a grid of icons. I pretty much use 6 apps. Maybe the answer is widgets. Maybe something else entirely. I'm just not sure the system is optimized for the way most people use their phones.
Perhaps fork iPhoneOS and iPadOS into separate entities. The iPad OS could make better use of all that screen real estate. Let them share a code base, but let each fly and become whatever is best for that system.
Multiple users on a single iPad signing in with fingerprints (like on MacBook Pro).
Being able to type to Siri... because you don't always want to (or can't) talk to Siri, but natural language is just so nice and easy. This is even more important on the Mac. I'd love to type reminders, timers, bookings, and questions right into Spotlight.
Smarter Siri.
More animated wallpapers (still only circles moving?!?)