Trying to brainstorm about some pitfalls the game could have, with the least amount of work involved. We want it so that the player isn't always guaranteed to be moving entirely 100% forward. Sometimes little tiny steps back.
Category 1: Monster Abilities
The Bandit Disarm skill I really like (at least for Black Bandits, original bandits I think are too early for that), but it's just a cool ability, it's not really a pitfall for the player (although it may force them to use a secondary weapon at least temporarily).
A really good pitfall would be that a Gypsy maybe at melee range can cast a spell that puts a -1 core bonus on your currently equipped weapon (i.e. a +3 becomes a +2, a Sword becomes a -1 Sword).
Maybe Skybabies have a chance to Weaken you with their melee attack which reduces your max HP by 1 (they need some identity).
And Bone Demons could have a chance to destroy an item you have equipped (weapon, or shield) upon attack.
Category 2: Item Badness
Examples of this would be an Increase Health/Strength scroll actually DECREASING the stat, like having a 10% chance to do so, and giving you a message saying "Something went wrong!" etc. Or maybe even you eat roasted food and it's like "Cough! You almost choke to death on the damn thing!" and it brings you to 1 HP.
Also examples would be involving sticky cursing, and negative Core Bonuses. This would require a creation of a Bless scroll or Remove Curse or whatever as well. Only thing about this is it sorta would involve item identification, which would be great, but might end up being a decently sized investment of time.
Category 3; Traps
I'm not sure how easy this would be for you. But some examples of traps are:
- turns a random item in your pack into rocks
- temporary strength decrease
- temporary confusedness (pressing one direction moves you in a random direction)
- exploding tile (brings you to 1 HP)
- Hole in the ground, suddenly moves you to the next dungeon level! <--- I like this one