I think co-owners should have more ability. If you are made a cow owner the owner must really trust you. Meaning, cow owners need to be able to ban people and lock and unlock the island while the owner is away making them able to keep the island running smooth. there is really no benefit from being cow owner right now. its basically a member. Another thing, friends and people of the same island should be able to access each others sell chests in my opinion, so that while one is away it can be restocked by another member of the island.
There is an option to lock, however letting co-owners ban players is not a good idea, it is under your control to decide who comes to the island. If you need someone banned, lock the island until the owner is on.