For Sybils, that’s something I’ve considered. I’m thinking of making voting eligibility a trailing indicator where it tracks past activity to be considered a ‘citizen’ of that community, and allowing the mod team to temporaily lock down the community if they see such a thing happening.
Unless you made your proof of work really hard, but then not a lot of people can participate.
What you need is a participation treshold to be hit before someone can vote. Kind of like HN limits your downvote until a certain karma threshold.
You don't protect against tyranny of the majority by allowing minorities to dictate policy, that's just regular tyranny.
You protect against majorities by requiring supermajorities for all the most important functions of government: constitutional change, impeachment, and censure.