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This was also my first thought - it reminds me of some Matrix based note taking apps.

See the Matrix blogs at https://matrix.org/blog/category/this-week-in-matrix/ to see how these projects have been progressing.

I think there is space for another paid competitor also built on Matrix!

Interesting. I'll take a peek when I have a second. From a quick skim, I see one notable difference: Kontxt not only enables rich web collaboration, but it helps atomize the web to re-use key pieces in secondary contexts like a social network or in the future, to create new content.

twitter could have been this, instead of the pile of garbage that it is today

That's one of the goals.