I mean, if you've unlocked every class, defeated Baal, gotten every item and endind...theres nothing else to do.
If your talking about item world, that's a different story, you dont have to max residents out to master this game.
Disgaea 2 the same way except for the Dark Gate and Land of Carnage. I've did everything there is to do in those games. Getting residents for items is just something extra you can do in your spare time.