1. The home screen on an iPad Pro should support widgets inline and should be able to be placed wherever a user wants within the grid. Android's been doing it for years. This doesn't necessarily mean more rows of icons and such (probably means less in actual practice).
2. The multi-tasking panel that you slide out from the side of the screen to do split screen is a horrid mess. It doesn't even have a quick app search feature! Its unruly when you actually want to use it and takes much longer than it should to find the second app you want to open. Its not intuitive and feels like a band-aid. Not to mention, no drag and drop from app to app... Horrible.
3. iOS does not have system-wide keyboard support. Users should be able to use the tab key on the home screen to tab through the app icons. Users should be able to switch the views on the home screen with a keyboard combo (CMD+Right/Left Arrow perhaps). Apple's own apps should have full keyboard support. Users should be able to navigate every view in every app with only the keyboard. This is a pro feature and a huge accessibility gain for disabled users.
4. The Apple Pencil cannot open the Notifications Pane, the multi-tasking pane, or the control panel. 3D press gestures (that would allow iPad users to access an app's widget on the home screen, just like on iPhone 6s and beyond) should be implemented. Its not used to its fullest potential and feels crippled at this point.
5. Lack of file system access or terminal based tools cripples iOS for software development.
iOS is not a "pro" operating system no matter how iPads are marketed. And I'm sorry but surfing the web and checking email or typing out a word document isn't "pro".
Now, saying all of that, the iPad Pro is a great consumption device and I really like mine. The hardware is solid. iOS for professionals... not so much.