If the maintainers are reading this is cool. I would start with an exporter/importer for pages from Wikipedia format into whatever format you use, it should also deal with the media. This is no small task but what you need.
It's legitimately a pain to do, because most Wikipedia articles use a bunch of templates. So for anything outside the most-simple of articles you suddenly have to implement a lot of the more painful bits of wikitext. Then if you want to import them back to Wikipedia, you've got to match those things back up or you'll get reverted for messing up the page...
Plus complying with CC BY-SA requirements for the content, of course.
Plus complying with CC BY-SA requirements for the content, of course.