1. Lots of people like realtime chat. It's worth having something that fills that niche (the old "mailing list + IRC" combo used to work well)
2. Forum software is usually fairly awful. I don't want to install some 15 year old pile of PHP with bad UX.
3. A "Stack Overflow" style site requires a lot of moderation and either annoys users for being too messy or annoys users for being too strict
4. I actually still have some fondness for Google Groups but Google's brand is too tainted for most people.
5. Nobody under 30 seems to use email any more so mailing lists are out but something that syncs with email is a must (reply notifications etc)
I am also hoping for something free but very easy to install. (SaaS with a free tier, one-click docker or similar). And easy on resources ideally (I guess $10/month for hosting is about our limit at the moment)
Yeah. I'm asking for a lot.