This mythical higher-end iPadOS doesn't need to be macOS, nor should it. This is still a touchscreen computer. But it ought to be powerful enough (especially on something like the larger iPad Pro) to at least be viable at replacing 90% of what a MacBook Air or Windows PC Ultrabook is capable of doing.
Improving iPadOS will not be enough to make it 90% as a MacBook or Windows ultrabook for most people, especially for work.
Simply because it would still run mobile apps.
The file app could definitely be improved, especially making it file centric and not app centric, allowing users to choose the default apps like on Android. But that's far from enough.
One key feature would be true background apps, like real time cloud syncing. Allow apps like Dropbox to run in the background 24/7 and update any change to files for immediate and offline use....
But even then, there are tons of desktop apps people use for work that have no mobile version or no full version on mobile. So other than running MacOS, maybe even including software like Parallels to run Windows apps, it will not be able to replace desktop devices for most people.
If you remove from the equation the Windows haters, who could be a majority on this forum, a Windows tablet is more versatile. Problem with those devices is X86 and its heat, noise and short battery life / stand-by time.
The new X Elite chips should address those issues. But again that would work for people who are ok with Windows like me, or actually even prefer it to iPadOS and MacOS, which you won't find many examples of on Macrumors for obvious reasons.
Currently the closest thing to what you look for is remote desktop...