I think that it would be neat to have some random little things to help explain mobs. Like a grave under a tree that was freshly disturbed, or a dark shrine in the middle of a lava pool deep in a cave, or maybe a small nest in dirt in the side of a hill.
This would add some cool depth to it. Especially if it had a light gameplay interactivity, say, digging up the grave/shrine yielded some items but causes more zombies/skeletons to spawn the next night.
My ideas, because I'm not leaving them in the howling wastes:
Some sort of megabeast that only starts appearing after you strike Adminium for the first time, or some long game metric such as that. Every few nights it occupies a random open underground location in the general vicinity of the player. I think it should be nigh-on-invincible, bronze colossus style: that way you have no qualms about running but beating it would yield more satisfaction. Diamond golem?
I'd love to see at least one more animal mob, preferably a combative one to change up the daytime a bit. Perhaps a wolf or something: harmless unless approached directly, but it attacks when provoked and yields fur?
And I'm going to throw on a question:
I made a secret doorway through a painting. But every time I enter the game, as long as there is a space behind the painting, it falls off the wall. Just wondering if that's intentional or a bug or something.