Anything considered obscene by the community works best. If that happens to be NSFW material of any kind, then there you go. If for one community ‘obscene’ means “bestiality and child porn” but allows erotic nudes, then that too is legit.
EDIT: Just realized that this was about suggestions for one forum, not for Discourse. @lee-dohm, you might be interested in joining and perusing the archives of this stackexchange community, Community Building. I think it will better help you for your current situation.