Add a monster spell that makes a nearby other monster berserk. The behavior for a berserking monster is unspecified, but presumably involves it attacking anyone (besides the caster?), or attacking the player, with no regard to whether it was previously peaceful or tame and no concern for its own health.

Arcane monster spell that reduces your memory of a single spell by a few thousand turns.

“Teleport armor” monster spell, which is a close-range spell that takes one piece of your armor and teleports it somewhere else on the level (it could be limited to a short distance to be nice).

Foocubi could cast this (but not cast any other spells), since they want to get your armor off.

A monster spell which does high area of effect damage, but clearly indicates what that area of effect will be a turn or two before it goes off, so the player can possibly get to safety.

There should be a spell (probably on the player’s side, possibly on the monster’s side) that dispels magical buffs such as protection and reflection. Or else, if adding a new spell is unwanted, this effect could be folded into cancellation instead.

A silence spell for either players or monsters or both. Not specified whether it would be area-of-effect or targeted at specific creatures. The main function of silence effects would be to prevent spellcasting. Not specified whether this would also affect reading scrolls (though probably not since you can “cogitate” the formula on scrolls if polymorphed into a speechless form).

However, this may require defining certain existing spells as not requiring speech, because it does not seem entirely fair to take away all of someone’s escape or healing spells with fairly low level magic.

New clerical monster spell (possibly also a player spell) that creates a region of black storm clouds between the caster and target and moves across the map towards the target. The clouds block vision, can strike anything in or near them with lightning (though not intelligently), wet items on the floor beneath them and in the inventories of monsters beneath them, hit things with gusts of wind if those are implemented, and can spawn temporary lightning monsters that vanish when the cloud does.

Another variant of this idea is as a static cloud that can be targeted like a stinking cloud, probably as a level 7 spell (depending on how bad the lightning is).

A monster spell (or perhaps some esoteric monster damage type) that makes you intrinsically fumbling for some time.

Monster spell that either generally or specifically debuffs the player. It targets beneficial timing-out intrinsics you have, such as speed, detect monsters, and see invisible, and reduces their time remaining, possibly removing them outright. Does not affect permanent intrinsics or extrinsics.

Additional clerical spells for aligned priests to cast: raise undead, floor to lava, ball lightning (not a ray and thus not reflectable), summon elemental.

Monster spell (or song, for bard-patch variants) that temporarily causes allies to grow up by one growth stage, regardless of its level (e.g. a little dog under this effect will become a dog, a demilich will become a master lich).

A new “illusion” or “disguise self” monster spell, which makes the caster or an allied monster appear as another monster of its choice until something disrupts the illusion (it gets attacked, otherwise takes damage, or it makes an attack). Monsters specifically cast it when you cannot see the target monster. It might be possible to implement this spell to be player-castable as well, but this would be less effective. (you could use it to fool monsters into thinking you’re another hostile monster so they don’t attack you, but not much else).

The illusion favors nasty monsters, but low-level spellcasters may only be able to create illusions of somewhat higher-difficulty monsters. A potentially evil addendum is that high-level spellcasters may use the spell to make themselves or other powerful monsters appear to be weaker monsters. Any bosses that use this spell may use it to make themselves appear as a weak monster so they can take you unawares.

Perhaps if you have extrinsic see invisible (and only extrinsic), you can see through such illusions.

Add a high level monster spell “raise dead”, which resurrects nearby corpses as hostile monsters.