I think the balance between “sovereign communities”, benevolent (?) dictatorships with one super-admin, or democratic collectives needs to be found. Ultimately this is something that needs to be hammered out, but any solution would be better than none.

Three possible solutions (just spitballing, not much thought put into them):

• What I described before as “federated collectives”. New communities can join a collective by asking the others. Maybe there will be a user-weighted vote on this or some other governance mechanism, or maybe it will be consensus-based. Communities can be kicked out of collectives by the same mechanisms or leave on their own. The collective can decide whether mutual mod actions are allowed or not.
• “Colonial-style” relationships. One “empire” community has powers over other “colony” communities. The empire’s mods can (maybe) perform mod actions on colony communities but not vice versa. Colonies can declare independence or the empire can kick them out. Colonies can join only by asking the empire to accept them.
• “Roman Republic collectives”: Mods (or active users?) of communities elect a board of prefects for the collective. Prefects (maybe) get mod powers on all communities. The prefects can vote to accept new communities or kick others out. Maybe they can get other management powers too. The “benevolent dictatorship” case is just a special case of Roman Republic where the number of prefects is 1

Of course, in all cases, an instance refusing to honour the powers of governance authorities would be interpreted as the instance admins withdrawing the community from the collective. Sort of like automatic defederation.