Build database of Linux/Windows executables. When user try to ran program, name of it and checksum are compared to values in database (part of database could been downloaded). KDE will download definition script, which will describe what to do based on Operating System, architecture, etc.
Install script could been downloaded and ran. Desktop files could been generated, etc. Why to force use some thing like launchers, while Desktop Environment is launcher by itself!
Launchers like Lutris usually have a bit more functionality, like setting a specific wine version and other options.
I think there are already various tools to download scripts from the internet. The issue is that those scripts usually are really hard to vet for security issues, malware and so on. Also it is really hard to give proper support for things like that, not sure if KDE could help with that.
I think, Lutris could also have security problems. You must only look at the solution in universal launchers, or just allow to create extension to desktop files and executable file properties. Also, allow to create new desktop file with altered settings from file properties dialog.
This could be just first step.
If you will, you can do other thinks I describe in this thread, like downloading shell script or other entity, based on file name and checksums. This entity could contains information how to launch software, based on system version, etc. Instead of script/some definition, you could connect with first part of this post (not full thread) and allow to automatically download altered .desktop file. But, this could be a second part.