Well, ideally, plugins should come from your needs, but sometimes, you see a cool feature you didn’t think about and decide that it would be good for your community.
I don’t know your needs nor your community and I won’t be of much help here, but I illustrate my example with this plugin: Who's Online Plugin (discourse-whos-online)
I didn’t think I needed such a thing, it never crossed my mind. I saw the related topic on meta and installed it on my forum. I think in my case (fairly small community), seeing other people online is an incentive to interact with the forum.