Wavebreak Hippocamp

Whenever you cast your first spell during each opponent's turn, draw a card.

Gandalf of the Secret Fire

Whenever you cast an instant or sorcery spell from your hand during an opponent's turn, exile that card with three time counters on it instead of putting it into your graveyard as it resolves. Then if the exiled card doesn't have suspend, it gains suspend. (At the beginning of your upkeep, remove a time counter. When the last is removed, you may play it without paying its mana cost.)

Slitherwisp

Flash
Whenever you cast another spell that has flash, you draw a card and each opponent loses 1 life.

Nymris, Oona's Trickster

Flash
Flying
Whenever you cast your first spell during each opponent's turn, look at the top two cards of your library. Put one of those cards into your hand and the other into your graveyard.

Lavinia, Foil to Conspiracy

Vigilance
Whenever you cast your second spell each turn, investigate. (Create a Clue token. It's an artifact with "{2}, Sacrifice this artifact: Draw a card.")
{T}: Add {C}{C}. Activate only during an opponent's turn.

Tidal Barracuda

Any player may cast spells as though they had flash.
Your opponents can't cast spells during your turn.

Faerie Tauntings

Whenever you cast a spell during an opponent's turn, you may have each opponent lose 1 life.

Glademuse

Whenever a player casts a spell, if it's not their turn, that player draws a card.

Brineborn Cutthroat

Flash (You may cast this spell any time you could cast an instant.)
Whenever you cast a spell during an opponent's turn, put a +1/+1 counter on this creature.

Stinging Lionfish

Whenever you cast your first spell during each opponent's turn, you may tap or untap target nonland permanent.

Raff Capashen, Ship's Mage

Flash
Flying
You may cast historic spells as though they had flash. (Artifacts, legendaries, and Sagas are historic.)

Temple of Cyclical Time

(Transforms from Ojer Pakpatiq, Deepest Epoch.)
{T}: Add {U}. Remove a time counter from Temple of Cyclical Time.
{2}{U}, {T}: Transform Temple of Cyclical Time. Activate only if it has no time counters on it and only as a sorcery.

Ojer Pakpatiq, Deepest Epoch

Flying
Whenever you cast an instant spell from your hand, it gains rebound. (Exile it as it resolves. At the beginning of your next upkeep, you may cast it from exile without paying its mana cost.)
When Ojer Pakpatiq dies, return it to the battlefield tapped and transformed under its owner's control with three time counters on it.

Ojer Pakpatiq, Deepest Epoch // Temple of Cyclical Time

Naiad of Hidden Coves

During turns other than yours, spells you cast cost {1} less to cast.

Errant and Giada

Flash
Flying
You may look at the top card of your library any time.
You may cast spells with flash or flying from the top of your library.

Dream Spoilers

Flying
Whenever you cast a spell during an opponent's turn, up to one target creature an opponent controls gets -1/-1 until end of turn.

Final-Word Phantom

Flash
Flying
During each opponent's end step, you may cast spells as though they had flash.

Dreamstalker Manticore

Whenever you cast your first spell during each opponent's turn, Dreamstalker Manticore deals 1 damage to any target.

Alela, Cunning Conqueror

Flying
Whenever you cast your first spell during each opponent's turn, create a 1/1 black Faerie Rogue creature token with flying.
Whenever one or more Faeries you control deal combat damage to a player, goad target creature that player controls.

Rattlechains

Flash
Flying
When Rattlechains enters, target Spirit gains hexproof until end of turn.
You may cast Spirit spells as though they had flash.

Dreamspoiler Witches

Flying
Whenever you cast a spell during an opponent's turn, you may have target creature get -1/-1 until end of turn.

Taigam, Ojutai Master

Instant, sorcery, and Dragon spells you control can't be countered.
Whenever you cast an instant or sorcery spell from your hand, if Taigam, Ojutai Master attacked this turn, that spell gains rebound. (Exile the spell as it resolves. At the beginning of your next upkeep, you may cast that card from exile without paying its mana cost.)

Breath of the Sleepless

You may cast Spirit spells as though they had flash.
Whenever you cast a creature spell during an opponent's turn, tap up to one target creature.

Geyser Drake

Flying
During turns other than yours, spells you cast cost {1} less to cast.

Cunning Nightbonder

Flash
Spells with flash you cast cost {1} less to cast and can't be countered.

Mischievous Chimera

Flying
Whenever you cast your first spell during each opponent's turn, Mischievous Chimera deals 1 damage to each opponent. Scry 1.

Narset Transcendent

+1: Look at the top card of your library. If it's a noncreature, nonland card, you may reveal it and put it into your hand.
−2: When you next cast an instant or sorcery spell from your hand this turn, it gains rebound. (Exile the spell as it resolves. At the beginning of your next upkeep, you may cast that card from exile without paying its mana cost.)
−9: You get an emblem with "Your opponents can't cast noncreature spells."

Glen Elendra Pranksters

Flying
Whenever you cast a spell during an opponent's turn, you may return target creature you control to its owner's hand.