maybe that you lose moral or luck or whatever when you attack another island, and you have to research a special thing like amphibious to be able to attack farming villages on other islands? it could be different for different worlds
I love the idea, but maybe 20% instead of 15? I could use this a lot, but that would probably fall in abuse right?
also, I think it would be better to not be able to attack other island's farming villages or send resources in this, but using my suggestion above, it would be fine in circumstances where you are not improving morale.