Just to add to the list of people to invite: Mastodon App Developers. That’s one of the groups that was also addressed in @rysiek 's proposal as having a todo (change default instance).
Second, I think a good step would be to make the topics of SocialHub clearer, e.g. continue along the lines of Adding a Moderation channel to the forum and Should we have a testing category .
Maybe, one should also consider an “Apps” category and a “first time federation developer” one. No strong opinions from my side on it though.
Another category that I was sometimes wondering if we should have is json-ld…