I Was thinking it would be a good Idea to put in spawning locations and times to NPCs. Examples below:
1. Ghosts, zombies, skeletons and demons ex. can spawn in dungeons, But only Ghosts and zombies could spawn in a grave yard.
2. Animals have some kind of den or nest that they could spawn from and or go to sleep at night/day if it is undisturbed by players.
3. Besides one or two hostile monsters, I think the only hostile mobs on the open surface should be the hostile animals.
4.Bats/Birds cycle where as the birds pawn in during the day then despawn at night where the bats would spawn in and despawn at sunrise.
5. Hostile animals and mobs attracted to dead mobs and players or maybe even spawn in some.
I was think if you put area based spawning system and not just a time/lighting one you would have a spawning system that would not only be unique but give the players a more engrossing experience but also a more realistic one.