I suggest the creation of a tool -- either a local gadget, or Apertium add-on, or something else? -- that adds these features to the Simple Wikipedia's editing window:

• color-coding of the text, based on word frequency, using the Basic English word lists and further word lists
• alternative word suggestions, based on a thesaurus, when words are not in the above-mentioned lists

Inspiration/context:

1. Upgoer6 is awesome. It "colours words depending on how common they are", using the frequency lists from: https://en.wiktionary.org/wiki/Wiktionary:Frequency_lists#TV_and_movie_scripts

2. Upgoer5 is also cool. It underlines words that don't appear within the list of the 1,000 most common words. It also offers alternatives to those non-common words, using this thesaurus: https://gist.github.com/anonymous/4645194 (per)

3. Thing Explainer by Randall Munroe is a popular and great book.
4. This tweet: [I] googled "what does python do," it sent me to the Simple English Wikipedia page, I've never been more insulted and educated at the same time
5. languagetool.org is also cool. A FOSS grammar checker in multiple languages. More at w:LanguageTool. More at T105153: Make the LanguageTool deployable to the Wikimedia cluster.

Notes:

• Simple English Wikipedia has some non-strict guidelines around word choice, detailed at https://simple.wikipedia.org/wiki/Wikipedia:How_to_write_Simple_English_pages
• There's a semi-related basic userscript at https://en.wikipedia.org/wiki/User:Phlsph7/Readability

Possibly related:

• T196354: Enable MT for "Simple English" as if it was (regular) English
• T103067: Enable Apertium Machine Translation for Simple English