Peaceful monsters casting spells do not interrupt multi-turn occupations.

Spellcasters who have summoned insects or nasties can sometimes also cast a spell that makes the summoned monsters explode at you.

Spellcasting monsters that want to go through a locked door can cast a monster-spell version of knock that unlocks it. Theoretically they should also be able to do this from a distance if lined up with the door, but monster decision-making does not decide to go through a non-adjacent door.

Sometimes Medusa’s Island is replaced by a new level Circe’s Island. Circe has a nasty polymorph touch attack (or perhaps can even cast it as a spell, at range), and may be accompanied by sea monsters Scylla and Charybdis.

Spectres should have some method of stealing your spell knowledge, then gaining the ability to cast those spells themselves.

Spellcasting monsters, at least the mid to upper range ones, should be able to cast sleeping at the player.

Assuming the monster and player spell lists are unified so that monsters cast player spells: When a mind flayer sucks your brain and causes you to forget a spell, it may gain the ability to cast that spell.

Add a monster spell, only cast at range 2 or 3 away from the player (not adjacent). A successful cast means the monster makes a single melee weapon attack against you. (Probably any passive attacks by the player should not happen.)

A monster spell ‘curse weapon’ or ‘curse equipment’, a clerical spell that is a weaker version of curse items: it only curses one item, but selects from among worn and wielded objects. If the target isn’t wearing or wielding anything, it will target one piece of equippable gear from their inventory, or failing that a random item.

Make fire giants (possibly only in the Valkyrie quest, possibly others) spellcasters who can only cast a fireball spell. Or, to avoid a flavor issue with giants being not very smart to be good at spells, flavor it as something other than a spell (it could also be implemented as a new attack type).

Add a new monster spell “grease” which inflicts you with slippery fingers.

Divine lightning (a god smiting you, particularly Moloch when you attack his high priest) should ignore reflection; it’s a “wide-angle” blast of lightning. Maybe this should apply to the lightning monster spell too. The high priest of Moloch should also be shock resistant.

Distinguish between monster spells “summon random monsters” and “summon nasties”, allowing lower level casters to summon at-difficulty non-nasties.

When you are hit by the aggravate monsters spell at melee range, you gain intrinsic aggravate monster for a little while.

Remove the monster spell destroy armor, and instead allow them to read destroy armor scrolls at you offensively.

If you attack a spellcasting monster that is currently at full potential to cast a spell, it may cast a counterspell to ward off your attack.

Scale the monster “healing” spell so that late-game spellcasters can actually heal themselves with it.

High level spellcasting monsters should have stone-to-flesh in their arsenal. Then they can use it on statues which come to life, always fighting on their side. They should also attempt to cast charm monster on your pets, with success inversely proportional to the pet’s level.

Nymphs are weak spellcasters and may generate with a few enchantment spells or have hardcoded ones like sleep and charm monster.

Dragons can cast spells.