Not everybody uses Emacs?

TeXmacs is, weirdly, completely distinct from Emacs. Both are, er, macro bundles, in the sense that they’re an interpreter with an editor in it.

TeXmacs is fine, but one difference is I can imagine laying an etherpad style coordination layer over this and getting something new.

TeXmacs is also completely distinct from TeX! It does use TeX-inspired layout algorithms, with modifications to make them better fit real-time typesetting as you type. It's also striving to be an environment for interaction with other tools, (not unlike Jupyter Notebook).

If you want a structured scientific web editor with collaboration, check out also https://www.fiduswriter.org/. Built on top of ProseMirror.

But Fidus is closer to "WYSIWYM" like LyX — structured editing with "acceptable" typesetting, and TeX export for high-quality typesetting. TeXmacs shoots for true "what you get" high-quality typesetting as you edit. Mind you, the edited model is structured in "what you mean" spirit (with quite good UX to "see" the structure), not flat "slap formatting on characters" model typical of WYSIWYG editors.

It runs in a web browser, so runs on tablets, cell phones, etc. but Texmacs doesn't. Also, it very directly uses the native LaTeX file format.