I try to run discussions and challenges in some of my communities, the problems are just me getting busy and general forgetfulness.
The womens stuff community does a great job of keeping people engaged with daily topics and weekly chat threads, which other communities could take notes from.
Beyond that, better mod tools, public calendars to schedule events and puzzles would be great for larger communities.
Personally, I think Lemmy needs more 'best of' posts, or maybe just awareness of the best-of communities, to encourage engagement and showcase the community.