It would be cool to have lots of harmless monsters that don't drop crowns, heat and mats when killed.
Also there should be some kind of penalty, when a player kills them and one should be able to recognize them easily by some kind of aura or whatever.
Examples:
When a player hits a friendly Poison Toad, he becomes poisoned.
When he kills the Funky Snail her shell sticks to him and he gets movement speed decrease for the whole level.
When he hits the Old Swearing Gravedigger he becomes cursed.
The Fire Bunny burns him when killed.
The Nymph follows players around and drops hearts sometimes as long as they don't cut a tree or bush for some time.
The Rainbow Cows fresh corpse attracts Wolvers.
The ghost of the Crazy Parrot haunts a player until Terminal or Subtown and switches his weapons sometimes.
The Wizard Spider can eat vials and capsules (dropped by players or monsters) and spit out better ones or different ones.
etc.
These monsters should run of, scream or cry when hit, too. That way people will learn about them faster.
This would make many battles more interesting. The presence of harmless monsters would turn many simple hack-and-slay situations into some kind of challenge.
I always liked the idea of passive monsters. Perhaps there should be like a little puppy that, if you don't kill it for a long time, increases your attack (perhaps low at best) and, if you kill it, it attracts alpha wolvers.