Funny or not, your idea is not too far-fetch. Nation defense is not based on the country girls were born, but the current country they currently active. There maybe an event like plant more trees and flowers around restored Kodaibana, fix Bremen to its original glory (or at least, attempt to), guard the city from heavily invading pests. Even the main quest seems a little bit more serious now with super powerful pest (that sadly, you won't fight them directly) appear to cause havoc in each countries.