Same for themes
The would open up a can of worms.. I suggest to keep it as strict as possible, preferrable a ClassicPress repository for themes and plugins.
I integrated that in the plugins in the past https://github.com/afragen/github-updater
So maybe explain better what do you need because is already possible to use a custom solution for update plugin/themes outisde the official repository.
Agree, it can already be done. Without description of what the idea is, I don't understand.