TeXmacs is as extensible as Emacs, being based on the same principles: having a full-fledged scheme interpreter to modify the behaviour of the UI and to program

TeXmacs is as extensible as Emacs, being based on the same principles: having a full-fledged scheme interpreter to modify the behaviour of the UI and to programmatically edit the documents. You can even have Emacs-like keybindings :). What Emacs lacks and TeXmacs has is a sophisticate typesetting engine (i.e. something like TeX) to render complex documents. What Emacs has and TeXmacs lacks is a large user base and dedicated people which develop extensions, but it has a small number of core developers which maintain and use it everyday. It has ~20 years of development history.