A Quest where instead of finding your nemesis on the last level, you kill them at the start or very early in the quest, but they repeatedly respawn like the Wizard of Yendor until you fulfill the quest which stops them from respawning.

You should be able to wear only one of a pair of gloves, getting half of its benefit, but allowing you to wear a different type of glove on the other hand for half of its benefit.

Add a “classical mechanic” monster as a counterpart to the quantum mechanic. It has a unique attack strategy; if it’s not in melee, it will rush towards you all at once and send you flying when it collides. This is an elastic collision that conserves momentum, so how far you are sent flying depends on how far it traveled to reach you and your relative masses.

If it is in melee range, it has a melee attack that drains your magic energy to make itself stronger, but if you subsequently kill it and eat its corpse, you regain exactly as much energy as you lost (conservation of energy). They also mutually grudge quantum mechanics.

There should be more birds in NetHack, particularly weird ones which could have nonstandard AI, such as peacocks, birds of paradise, and Canadian geese.

A quest that is filled with Chinese lore and new monsters from Chinese folklore. Possibly a variant of an alignment quest, or a straightforward implementation of the Monk quest, or a Noble quest for a race which doesn’t already have an overridden alternative Noble quest.

Magic markers are susceptible to water damage, getting their charges reduced when they get wet. This could be either a few charges, or could possibly go up to the level of halving the remaining charges.