This is an idea I had for a comments system to replace utterances for people who don't like GitHub. I'm not claiming to have all of the answers from the get-go, but here's a rough idea of what I'm thinking of.
It's a portmanteau of "common" and "comments". Yes, it sounds dumb, but I can't come up with anything to better describe this project. It's a common platform for you to host comments on.
That sounds repetitive. Let's stop bickering about the name and get on with the description of how it works.
So I'm imagining a plug-n-play system that handles more than one Source of Truth. For instance, it could have a module for Gitea, a module for in-house storage, even a module for GitHub if you really feel like hooking Commonts to GitHub (like that makes sense). There's still a single Source of Truth, but you get to choose which one.
Comments can be added to a Commonts instance in many ways. A form will be placed at the top of the comments where they're displayed, but much like the Source of Truth, we can add plugins to take comments from elsewhere. For instance, you could add a Fediverse integration to take comments from the fediverse (mentions to a bot account?).
That's my favorite part of this: the person running the blog gets to decide! The comments are injected into the page via JavaScript, with standard classes that the blog owner can style however they please. Of course, these class names would be definable on an instance-wide basis (so, for instance, a Commonts instance on tilde.team could use class names for tilde.team standard CSS by default).
Well, since Commonts doesn't exist yet, I'll include an utterances thing below for comments. Let's make something great!