How would you do that? Debian would not create and maintain a “core debian” variant just to be installed then receive the extra packages.
To be honest, I do not fully understand your question here. Could you rephrase?
Would the *ubuntu packages replace, instead of add on top of default debian packages?
They must not replace. If they are merely installing KDE on top of Ubuntu, then theres nothing to do here. The work is already done for us. But if it is doing more than taht, then they should be different packages building on top of the default debian packages for KDE et al.
Sort of like how LunarVim is a distribution of NeoVim. It is the same NeoVim, but with pre-configurations and plugins shipped OOTB, and it can be packaged separately.
What about dependencies?
Thats the beauty of this. Package managers are already equipped with dependency management. It is far easier to manage dependencies with a package rather than rolling out your own distribution. It is literally one of the biggest reasons why we use package managers to begin with. We dont want dependency hell!
Would debian have to coordinate with all *ubuntu maintainers (and they too, between them) for compatibility tests every time debian needed to update one of its packages? Or they’d just update and *ubuntu would have to scramble to release fixes for what had been broken?
This is a debian specific question, so I will try to answer more generally. It would just have to be done in the same way any package is maintained on that distribution. And this varies by distro; some distributions have different workflows for their package maintenance. The point is that we make use of these already defined workflows that have worked for decades and been iterated on. It is much easier to package than to create a new distribution.
Not to mention convenience; would you have to download debian, download *ubuntu, install debian, then your *ubuntu?
Instead of installing *ubuntu, you install Debian, then run one command: sudo apt install *ubuntu. I see these as nearly equivalent. Moreover, it could be made to be an option in the distribution's installer, sort of like EndeavourOS and Fedora do it.
Why not then package the “core debian”, with the tested component versions that work with the *ubuntu packages you’re downloading?
That can be what I mean with it being an option in the installer. But if you mean maintaining a whole separate distribution just for this, well ... you are maintaining an entirel separate distribution just for this ... instead of just maintaining a package.