I would like seamless interoperability between CLI and GUI apps, a unified file system, the ability to run virtual machines, containers[1] or maybe just nginx[2].
And then you have the usability issues of the iPad(OS) like external displays being mirrored only, or only recently being able to actually access files from a USB drive...
The iPad is a great media consumption device, that has some limited professional use (art, video editing maybe), but let's not fool ourselves that it's anywhere near being a replacement for a fully functional computer.
In the "mobile app ecosystem", each application is more or less an island. It would be possible to achieve something similar on iOS/Android devices, but the entire "value proposition" of walled garden ecosystems isn't compatibel with this idea of open and creative computing. One prerequisite is that I actually own my device and can do anything I want with it, without the platform owner getting in the way.
The main limitation is the interaction model, and that's just down the the mechanics of touch interfaces. Firstly fingers are imprecise big fat squishy blobs. Secondly the tablet form factor doesn't afford a large physical keyboard. These are the impediments and they're just facts of the form factor. the new multi-tasking interface in iPadOS looks like a great step forward, but it's always an issue.
I have good news, iOS user's productivity woes are solved. Using iSH you can pipe text between command line tools all day every day. Finally the power of the iPad is unleashed!