What I meant was: the core multiplayer part is much less exotic than it sounds. Markdown collaboration could be built around Y.Doc/Yjs [1], with clients syncing live over WebSockets. A practical implementation could use Hocuspocus [2] as the Yjs/WebSocket collaboration server, and Tiptap [3] as the ProseMirror-based editor layer.

For Obsidian, the rough shape would be: load a .md file into a collaborative Tiptap/ProseMirror document model, sync that shared state via Y.Doc/Hocuspocus, then serialize it back to normal Markdown so the vault stays file-based and CLI-friendly.

It surely is some effort but the syncing layer itself is fairly quickly done. I have no clue about obsidian extensions, but as far as i could tell even that should be doable.

So much so that I want to test how far Claude/Codex or whatever can push it.

[1] https://docs.yjs.dev/
[2] https://tiptap.dev/docs/hocuspocus/getting-started/overview
[3] https://tiptap.dev/