Suggestion
The way some monsters behave or are affected by in-game elements should be changed or added to. For example:

* Shadows, slow shadows, shadow lords, pixies, pixie archers and will o'wisps trigger traps, and it feels they shouldn't (shadows are just that, the other ones creatures fly).
* Acid and lightning vortices should probably destroy webs like fire ones do. (Acid and lightning bolts already destroy webs in 1.2.0.)
* Jellies, plants, will o'wisps, living walls and wall beasts can bleed and that feels a bit weird to me (your mileage may vary).
* Shadows of all kinds can be pickpocketed. Literally stealing from a shadow sounds a bit weird.
* Wands of digging and Mystic Shovel could be made to damage living walls, wall beasts, stone golems, stone statues and (greater) earth elementals, in a somewhat unusual application (like Bless damages undead).
* Throwing potions of water (perhaps there will be more methods to actively damage monsters with water in the future) should rust iron golems and do some damage to them.
* Ghosts of any kind can't cross rivers, and they probably should be able to.
* Monsters that phase through walls can currently only do it through diggable ones (this shows in the dwarven graveyard). Let them pass through anything.
* Shadow wyrms should be undead. They can summon shadows and have an aging touch.
* The greater white unicorn will usually attack chaotics on sight, and will never harm lawfuls even if they hit her first. I don't see why she shouldn't fight back against someone with obviously ill intentions just because they share the same alignment.

Why it'd improve the game
Enhancing the level of detail even more :)

Notes
N/A