What exactly would change ? Only thing I can think of 'normal' users using is ctrl-c/ctrl-v which already work in KDE (even ctrl-win-arrow for virtual desktops could be configured without a fork.)? Different icons/shortcuts wouldn't require forking KDE...Not sure what else you'd change?
If I sat down to compose the list, it'd be full of lots of small things. Think how under KDE, the file copy progress UI is a semi-persistent notification banner instead of a normal dialog like it's always been on Windows. While each of these differences on their own aren't likely to pose issues, in aggregate they can give an impression of the DE being more unfamiliar/alien (and thus, daunting) than it actually is.
I would probably also overhaul the settings app. For example, the whole Appearance & Style group and its two drill-down sections could be pretty easily reworked into a single panel that directly surfaces the most pertinent settings while tucking away the rest under an "Advanced…" button that opens a modal — with themes for example the average user will at most be interested in changing the global theme. Only advanced users even know the difference between window decorations, application styles, etc much less want to be able to change any of those individually.
Hmm - I'm not sure those sorts of things would require a fork per se, but I can see making it more familiar to windows/mac users could be a good thing. I was just curious as I was a big KDE fan back when I used Linux full time, and wondered what had changed.
The two biggest issues I'd have with Linux full time would be audio and video. I haven't even attempted to run Linux audio in like 5-10 years - it was always so titchy with multiple 'standards' to configure. Video was not quite as bad, as I don't game much so I just needed basic functionality. But now I have multiple 4k monitors, high dpi, and all that jazz, I don't know how big a pain it would be. I just run linux in VMs (standard device drivers and setup) or ssh into a linux server for now.
A fork wouldn't be explicitly required, but generally in established FOSS projects, when attempting to make changes that are either large or widely peppered throughout the project you're going to be fighting against headwinds from people who are happy with project as it is, which is understandable. You're likely going to find yourself spending more time on negotiating and politicking to get changes committed than actually making changes.
Audio/video has more to do the distribution being used than it does one's desktop environment, though the DEs can do things to smartly design configuration UI and surface the most commonly used A/V settings in the right places.
