I've been looking at the ProseMirror as a way of building this meta-editor that could have plug-and-play components. The issue is, although the library is open-source, there hasn't been many open-sourced components that people have built. Ideally, we could go one step forward and abstract away the npm library; then everybody could build their custom editor by mixing components. One editor that sort of does this is StandardNotes but I haven't tried building something for it yet.
[1] https://prosemirror.net/ [2] https://standardnotes.org/extensions