I think the most obvious one is moderation. What gets deleted, who gets kicked out. Then for example community rules, what's the topic and rules of discussion. Every user/member could have a say in that. Maybe we could do some more structural and organizational decisions.
It gets a bit tricky with technology. Ideally we could do things like democratically decide to have a voice chat (if that's what people want) and somehow 3 months later the platform has a voice chat... But it's not that easy, software development doesn't work this way.