When a nymph steals an armor item, she remains frozen for as long as it takes you to remove it, thereby preventing her from stealing all your other things while you are removing it.
If you are polymorphed into a nymph and are lawful, you suffer an alignment penalty for stealing items from other monsters.
In order to prevent nymphs stealing your entire inventory while you disrobe, make their charm-steal attack not steal anything if you are immobilized due to taking off armor. Or make their armor-removal immediate like a foocubus, or give you a chance per turn to resist the charm, based on a Wisdom save.
Cut down on the list of inediate monsters by defining some of them to be either herbivorous or carnivorous (note that the game uses carnivorous to mean “can eat meat” rather than “only eats meat”).
Herbivores:
Carnivores:
Nymph race very similar to dNetHack’s yuki-onna, but based on regular NetHack nymphs (so no association with cold and no iron weapons). Has the same unarmed-attack-automatic-stealing ability that yuki-onna have, and has infinite carry capacity, but low HP.
Water nymphs may create a pool on their spot when killed. Items they stole from the player fall into it, of course.
Wood nymphs may create a tree on their spot when killed, and mountain nymphs may leave a boulder.
Fire/flame nymph, n, which normally only spawn in Gehennom (or possibly near lava not in Gehennom) and are noticeably stronger than other nymphs. Have two steal attacks which burn any items stolen, and also a fire attack. Potentially a passive fire attack as well. They also explode into fire when killed.
Fairies, a n monster similar to pixies: very small, flying, capable of hiding under objects. However, they are lawfully aligned and (?) always generated peaceful. If you chat to a non-hostile fairy, it will heal you a moderate amount (as in the Legend of Zelda) and become cancelled (a cancelled fairy can no longer heal you).
Some sort of late-game nymph monster that steals items. Helps mitigate an underlying issue: some of the early-game threats like stealing are completely absent in the late game, with no particular reason for that being the case.
Pixies, an n monster that can hide under objects. Chatting to them will make them say “Hey! Listen!” as an Ocarina of Time reference.
Message for when a nymph finishes stealing something from you: “The [nymph] giggles and vanishes.”
Nymphs are weak spellcasters and may generate with a few enchantment spells or have hardcoded ones like sleep and charm monster.
