Trying to make sure I understand clearly this long thread here.
Basically you would like to build an architecture equivalent to Ubuntu's PPA that users could add to their apt sources to get newer versions faster instead of dealing with their apt preferences?
I personally like the current idea of having a full distro with packages battle-tested with eachother and I'm a bit worried of the added time spent dealing with bugs between a package and a dependency that would be a bit too old, introducing unstable/unsecure behaviors into stable.
Maybe another way to do would be to have more frequent full releases of the entire distro to stable? More frequent releases could make the freeze needs shorter as problems wouldn't have as much time to accumulate maybe?
We do have more and more packages tested through debci, so making the need of autopkgtest more strict to up to testing could make testing an easier ground for more frequent releases maybe?