While we decided not to include MediaWiki to the list of featured projects, and while that was a good decision indeed, we should aim to include Wikimedia's flagship software project one day. Maybe not easy, but neither impossible. Maybe not for the entire codebase, but maybe there would be a way to identify areas where mentors and first tasks would be available.
There is the hypothesis that a project that is good to receive contributions for newcomers is also good place for regular contributors. Having more newcomers using MediaWiki documentation and contributing to this codebase must be a good mid term investment beyond the newcomers themselves.