Premonition - You can play the next X turns without any consequence to try out strategies, like in the Next movie. Random seeds should be kept so the results are the same afterwards.

If the game has items to collect:
Multiplication - You can multiply items before collecting them.
Self Multiplication - You can multiply yourself.
Alchemy - You can convert items into another kind of item.
Invisibility - Nobody can see you.

What about:

Rushing River - Carves out a randomly generated river flowing from one side of the room to the other (and then add creatures like spirits or vampires that can't cross running water)
Collapse - Pull down the ceiling in a room with a chance to kill creatures within a certain radius. This one would be strategic because you'd have to make sure you weren't caught and that you weren't blocking your way out.
Phasestep - For a cost of health and mana per turn walk through solid objects, damaging them in the process (enemies included). Useful if you've blocked yourself in collapsing ceilings.
Treacherous Ground - Convert floor tiles to mud, lava, silt, etc. (after N turns so that enemies can move & it's not just insta-kill). Can act as a portal for monsters from lower levels.
Wyrmlike - Player morphs into a human-wyrm hybrid and can "stretch" around targets (like those old Surround/Snake games). Hitpoints get divided up between cells, however, making the player more vulnerable the bigger he gets.

Fatal Fog - Breathing and sighted creatures take continuous suffocation damage and are blinded, moving randomly
Stardrinker - Opens a gravity-based vortex in the center of the room. Everything not nailed down goes in, including the player (strength check to hold onto something or be behind something immovable). Intangible creatures ignore, ranged attacks curve around and into the vortex, etc.
Boneshatter - Damages monsters with skeletons, turning them into amorphous blobs. Partial effects can cause a monster to be immobilized or drop weapons. Shapeshifters can still attack from this state.
Headstrong - Monster becomes so enraged it rushes X steps in its facing direction (hopefully into a wall or off a cliff). Creatures that collide can damage armor or hit each other with weapons, even on criticals impale one another.
Open Sky - Transforms the ceiling of the dungeon into a portal, admitting moonlight, rain, sun, lightning, etc. Great for pesky vampires and trolls that hate sunlight.
Future Self - Teleports a randomized, higher level version of yourself back to you in time. The future you will have better gear, spells, etc. and may either fight alongside you, jinx you out of annoyance, be an enemy (as you've turned evil in the future) or simply curse you and disappear.

Disguise - disguises self as targeted enemy making them think you're an ally instead.

Take Control - turns target enemy into an ally. Target will then attack your enemies even if they are of the same type as the target.

Death Touch - Targeted touched enemy instantly dies. Requires as much mana as the enemy has hitpoints to cast. If the player does not have enough mana the player takes damage equal tot he target's hitpoints instead.

Trickster - Confuses enemies. Some may start walking in a random direction, others may start attacking each other, etc.

Midas Touch - Turns targeted object into gold. If target is creature same mana rules as Death Touch apply and possibly risk turning yourself into a gold statue if you have insufficient mana.

Summon Creature - Choose amount of mana to vest into the summon. The higher the mana vested the stronger the creature summoned.