It should be able to “carry” comment threads in from wherever they’re happening, like the antithesis of Disqus. If someone, instead of first signing up and choosing route their input through the anointed commenting service, is right now discussing it on Twitter/Fritter/Mastodon/HN/Reddit/etc, then the commenting service should be able to cope with that. Doubly so if the original author is also on those platforms and responding. The idea is that the author should be able to highlight/engage the discussion wherever it’s happening and not lose anything over a reason as dumb as “not posted here”.
For a commenter, the service would enable you to discuss it in the venue that’s convenient and still have a shot at it being part of the canonical discussion. (But no one should be entitled to it. See below.)
For an author, the service would act as an aggregator/dashboard for all the platforms where a supported backend exists. It would reveal where the article is being discussed across the Web—even the parts not built by and inhabited by fediverse activists—and assemble them into a big, unfiltered metathread. The author would be able to sign off on individual subthreads, and they’d get syndicated into the canonical comment section. If the author has an account on those platforms, he or she would be able to link those accounts so that subthreads can automatically be blessed by virtue of the author having participated in it (or liked it).
The lesser (long tail) value proposition of a service like this would be to offer powerful tools for tuning your defaults. Maybe comments in /r/programming are by default killfiled unless the author explicitly intervenes to highlight that subthread, while people you’re following or with whom you share an org on GitHub are able to have their Twitter comments automatically approved.