A spell or other magic item or ability that can be used to apply an effect to a monster that prevents them from teleporting, levelporting, or warping.
Artifact whose carry effect deflects incoming status ailments onto nearby monsters. If there are no other monsters around (or if there is one monster who is causing the ailment and is ineligible), you take the status effect like normal.
Hitting monsters with a non-cursed unicorn horn cures them of status afflictions. Hitting them with a cursed horn gives them status afflictions.
Allow monsters to hallucinate (mostly from the same sources that cause the player to hallucinate, e.g. exploding black light). Hallucinating monsters will, after choosing a target, refuse to attack it 90% of the time (because it sees it as a fellow hostile monster). They also will not pathfind to items on the floor because they cannot see them clearly.
When a monster is about to wake up from sleeping or unfreeze from paralysis and you can see it, give a message to that effect.
In the absence of monster hallucination being implemented, make a monster stunned whenever it would become hallucinating.
Some monsters can block being detected by your telepathy, either innately (disenchanters, perhaps) or as a monster intrinsic conferred by armor (such as a tinfoil hat) or spellcasting by an ally. Also, possibly allow for area-of-effect “telepathy dead zones”.
Farlooking monsters tells you whether they are asleep, paralyzed, or fleeing.
Monster hallucination, if implemented as a status, should be rather stronger than confusion: when confused, monsters can still see what the other monsters around them are, but when hallucinating they ought to see the same random monsters as the player does, and lash out at any of them.
Make monster sleep work more like player sleep, where being attacked / taking damage is quite likely to end the sleep. Then sleep is not basically a duplicate of paralysis.
Make monster stunning work like player stunning: it moves randomly and may cause them to miss turns.
“Omnicidal” monster attribute: it attacks peaceful, hostile, tame, and possibly other omnicidal monsters. Suggested in relation to monsters reading cursed genocide or creating monsters.
Attempting to move healers out of the petmastering role: they now have skilled medical knowledge that enables them to hit monsters in melee with a scalpel or knife to inflict a “bleeding” status effect which will cause the monster to lose HP continuously. Any role is able to do this with a scalpel, but the effect is much more limited than it is for Healers. This means that for a Healer, combat now consists of hitting a monster to make it bleed, then disengaging and keeping away from it in the hope that it dies on its own. The Staff of Aesculapius no longer drains, instead it heals the damage it would have dealt (unless the Cyclops has it, in which case it drains life like normal.)
Additionally, the Staff of Aesculapius no longer has level-draining powers. Instead it has a drain HP attack that heals the wielder.
Invisible monsters wearing a mummy wrapping show up as an I (you don’t get to see what they actually are), but an I that moves and doesn’t persist on the map.
Invisibility works completely when the invisible monster and the detecting monster are far away. When within 5 (or 4?) squares of each other, its location is known to within 1 space (fuzzing the real location by 1 space in all directions). When the invisible monster takes a direct offensive action, or with some probability when it is adjacent, its position becomes known. It still appears as I, but the I will move wherever the monster goes until it becomes 5 or more spaces away, at which point invisibility once again works fully. This could work differently if the invisible monster is stealthy.
See invisible operates on a scale that determines what level of invisible monsters the player is allowed to see. (A low-level monster that drinks a potion of invisibility should be visible even with weak see invisible, an arch-lich casting disappear should require very strong see invisible.) Invisibility should do this too and have a scale as well (one notable place on the scale being that invisibility is kept only until the invisible thing takes an offensive action). If rings of invisibility and see invisible are made chargeable, this is an obvious avenue for them.
Monster status “discord”, which causes a monster to be seen as a target and be attacked by other enemies. On the player, discord is the same as wielding Stormbringer. Discord is available through the spellbook of discord.
