We'd need improved shipyards for kingdoms, maybe that only members on tag can move ships inside, and only ships built in that kingdom can enter. Someone who isn't a member will get a neutral ship from a kingdom dock when building one. Also, there should be ways to anchor ships or lock them in some way.
The possibility of leaving everywhere is making things much better though, imagine an attack by sea on a fort... wait, shouldn't they be up sind quite a while now?
