Navigation popups as we know them are in big part, or even chiefly, about patrolling actions (and other maintenance like disambiguation): see for instance w:en:Wikipedia:Tools/Navigation popups#Features. Are these going to be in scope for this extension on a later stage, after the basics currently mentioned on the page?
If yes it will be useful to ask contributions/suggestions to the maintainers of Navigation popups in various languages, LiveRC (mainly French and Italian), RTRC (mainly Commons), FastButtons (mainly Spanish) and Ajax sysop (mainly cross-wiki). A project idea with a brief outline of requirements/theory of change could be collaboratively edited as a section in Mentorship programs/Possible projects.
After all, what all these have in common is the aim to Make MediaWiki more AJAXy.