I wish there was a plain text format as base that everyone agreed on no matter what UI or backend is used; that would suddenly make it usable in any text editor and people could build tools and plugins that "just work" no matter whether Jupyter or something else is used.

The closest we got was the org-mode file format with human-readable data for everything, but it seems tightly coupled with Emacs unless you only want to use it as Markdown replacement.