Quicksand, either as a trap or terrain type (they both have advantages and disadvantages); stepping in makes you start sinking in like with lava, and if you don’t find some way to get out in time, you suffocate to death.

Carrying a substantial number of cursed items in your inventory has some denigrating effect on you. One suggestion is that enough cursed items counts as a negative luckitem similar to a cursed luckstone.

Barbarians do not require a saddle to ride tame monsters.

A compass item, which when applied points up or down depending on where the stairs to Vlad’s Tower are. It spins around if you’re on the level itself or inside the tower.

Various forms of AoE monster-debuff candles. Charm them, anti-undead, etcetera.

Special candle found generated in Gehennom. While one is lit on a level, it suppresses spawn rates and makes the difficulty about what it is now (without the candle lit monsters will generate at a much higher difficulty). They should be fairly nonrenewable, perhaps wishable but not polymorphable. This limits your duration of time in “easy Gehennom” before it gets harder. The candles work the same outside Gehennom, but are less useful (maybe they specifically suppress demon spawns).

Wearing an opera cloak allows you to sing like a musical instrument (e.g. the passtune).

Sell price depends on Charisma, whereas buy price does not. (Slightly more flavorful.)

New conduct for not hitting things with thrown items, and another conduct for not casting offensive spells.

Asmodeus and Baalzebub demand gold proportional to their difficulty or monster level, not the player’s wallet. Possibly, if the player does not pay, they remain on the downstairs and will not move until the player has paid, which allows the player to return with the requisite amount of gold (and may charge interest for the time you spent keeping them waiting). However, they will get angry if you get below their lair in any way (possibly measured by testing your lowest level reached each time you enter the lair).