> I could throw together a spreadsheet format that is all text. […] The spreadsheet GUI then becomes a advanced text editor that, when editing that particular format, exposes advanced content-aware controls not at all unlike advanced text editors like emacs can for s-expressions.
(Emacs also has an even more spreadsheet-like mode; ses-mode, but that saves its data in less-than-purely-textual format. Or, at least, less textual than org-mode.)
Emacs already has that, included in org-mode: https://orgmode.org/org.html#Tables
(Emacs also has an even more spreadsheet-like mode; ses-mode, but that saves its data in less-than-purely-textual format. Or, at least, less textual than org-mode.)