In order to check the readability of the content of a wiki page, one currently needs to use external tools like readability-score.com or the Hemingway app.
It would be convenient to have an interactive tool that can be invoked more easily on a given wiki page, so as to avoid constant back-and-forth copy-and-paste between the edit window and a third-party tool. This could also potentially work in VisualEditor, if we wanted.
The code behind readability-score.com is available under a free license, and there's also a JavaScript port. This tool is specific to the English language, but I seem to remember attempts to develop formulas for other languages as well.
Improving readability using those tools is done every week to write (and simplify) Tech News. Other areas of technical communication (like T90263: Improve glossaries to better help Wikimedia editors understand technical terms) could also benefit from this.