I'll start with villagers, they are able to trade with any player in Skyblock. This is rather unfair since that means anyone can abuse villagers. So I suggest you make that impossible for outsiders to interact with mobs on other islands (villager trading mainly).
Next is the ability to co-op. The ability to co-op someone should be restricted to the perm menu island owners have. (To give members the ability to do something; Ex. Change island biomes, kick/invite members, etc). The island owner should have the co-op ability and be able to give the ability to other members