Hmm, let me think. Starting with general OS/global UI improvements...

1. Fix the random forced app shutdown introduced in U10.
2. Add cursor keys or some other navigation method to the default keyboards. In all language variants.
3. Implement proper cut-and-paste that would work consistently the same way in all apps.
4. Bring back the status bar or at least put on a single screen all the info the status bar would have normally shown. Having to swipe all over the place to see the clock, connection status, signal level, battery level etc. is quite annoying.
5. Merge the lock screen with the notification screen to turn two pointless screens into a single sensible one.
6. Stop app covers moving randomly around the home screen. Ideally let the user place them or at least keep them in one place.
7. Fix the pulley menus for apps with long scroll contents. I have to give Jolla some credit there as they cleverly avoided pulley menus in the stock browser. Something similar is desperately needed in all apps with a lot of contents: email clients, text editors...
8. Allow the use of some sensible icons for the Launcher folders.
9. Allow the use of gaps (empty space in place of an icon) in Laucher.
10. Implement a proper file picker for the use in any app, with a particular focus on making the access to the SD card easier.
11. Skip the pointless <SD card ID> in the path to the SD card contents.
12. Add a "new voicemail" notification.
13. Have a proper dbus or similar mechanism so that tapping on a file in any file manager opens the file in an associated application.

...continuing with specific apps:

1. Display the number of characters/messages when typing an SMS.
2. Implement an option for faster email deletion. Ideally to delete them right away instead of moving to Deleted or have a tickbox to purge Deleted on exit.
3. Delete emails properly, including all folders with downloaded attachments.
4. Fix/implement proper email folder handling:
   a) Let the user order them or at least make the order consistent across accounts
   b) Allow the email client create, delete and freely move contents between folders, including across accounts
   c) Implement folder subscription
5. Do not show the emails preview when you have multiple accounts. Show just the list of accounts on starting the email app and either do not show the previews at all or leave it for until after the user enters one of the accounts. At least make it optional.
6. Add a "make link" to File Browser options.
7. Add bookmarks to File Browser.
8. In the absence of #13 in the above list, have an "open with..." option in File Browser.

I could think of dozens more. These are just a very brief shortlist I bashed out in a few minutes.