A few random thoughts here:
* some better tools for searching, tagging, tracking my history, etc to allow me to better use and self organize their docs on 'mdn plus' would be a useful service. it may also give them some more insight on how people are using it and what areas are of bigger utilization.
* tools to allow me to ask private questions in areas of their docs, with some prioritization for answers/responses.
* IDE plugins - I think there's one for IntelliJ Ultimate, but not specifically from MDN. Having more direct access to MDN info while inside my IDEs would be something probably worth paying for. I suspect they may get some useful data on when/where people are having trouble. I wouldn't even mind contributing some of my source code snippets back from within the IDE if it would help give more examples to MDN.
* a couple years back, a friend started a podcast experiment of just reading MDN docs for javascript. It was... a bit different, but trying out some other formats for their docs. perhaps a Q&A podcast where mdn folks answer some of the answers (taken from the paid-Q&A bullet point above). (but yeah, they fired a bunch of people last year, no?)
$2-$3/month, if there were/are nicer tools to help me track what I've seen, perhaps letting me add private annotations so when I come back I can jog my own memory... I would try that, at least for a while (I think I'd get use out of that). $10/month - for just some 'premium' content? Probably not. $50/year would, for me, probably be an upper bound (unless/until I see more potential benefits).
EDIT: had not seen 'down below' - they have 'Bookmark and annotate free and paid content for reference across devices.' as a feature - possibly along the lines of what I was thinking above. Signed up for waitlist.