An invisible displacer beast that you can’t see should not be able to use its displacement abilities in combat with you. (If you have see invisible, it should still be able to use them.)

By wielding a floating eye corpse and either applying it at monsters or hitting monsters with it, you can paralyze the monsters, provided they can see you. As a potential downside, you might paralyze yourself if you stumble or fall into a pit while wielding it while non-blind.

Displacer beasts may drop a hide, which can be enchanted or otherwise crafted into a cloak of displacement. (Or else this could just work with a displacer beast corpse which could be enchanted/polymorphed/crafted, without requiring a new object for the hide).

Instead of electricity blowing up your wands and rings, it makes you incapable of using rings (and potentially disables your rings’ extrinsics) for a few turns.

Make a radical change to the scroll of scare monster, making it no longer scare monsters on the ground; instead, boost its reading effect to a very powerful scaring effect, more so than can be accomplished with a horn.

Merge the potions of booze and confusion together, because booze is basically just a weaker form of confusion that also has a minimal amount of nutrition. It’s not certain which way they should be merged: deleting booze and favoring confusion has the advantage that monsters can still flavorfully throw it at you in order to cause confusion (you don’t get instantly drunk from being splashed with booze), but deleting confusion and favoring booze is attractive because it’s a more flavorful item in general, what with having a dual confusion/nutrition effect, being nonmagical, contributing to levels like the Wine Cellar, etc.

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).

When you have a slower pet leashed, slow down _ travel by resting whenever you would move far enough from the pet to pull on the leash.