Vanquish the Horde

This spell costs {1} less to cast for each creature on the battlefield.
Destroy all creatures.

Entrancing Melody

Gain control of target creature with mana value X.

Gather Specimens

If a creature would enter the battlefield under an opponent's control this turn, it enters under your control instead.

Luxior, Giada's Gift

Equipped creature gets +1/+1 for each counter on it.
Equipped permanent isn't a planeswalker and is a creature in addition to its other types. (Loyalty abilities can still be activated.)
Equip planeswalker {1}
Equip {3}

Mass Manipulation

Gain control of X target creatures and/or planeswalkers.

Dominate

Gain control of target creature with mana value X or less.

Elminster

Whenever you scry, the next instant or sorcery spell you cast this turn costs {X} less to cast, where X is the number of cards looked at while scrying this way.
+2: Draw a card, then scry 2.
−3: Exile the top card of your library. Create a number of 1/1 blue Faerie Dragon creature tokens with flying equal to that card's mana value.
Elminster can be your commander.

Elminster's Simulacrum

For each opponent, you create a token that's a copy of up to one target creature that player controls.

Inevitable Betrayal

Suspend 3—{1}{U}{U} (Rather than cast this card from your hand, pay {1}{U}{U} and exile it with three time counters on it. At the beginning of your upkeep, remove a time counter. When the last is removed, you may cast it without paying its mana cost.)
Search target opponent's library for a creature card and put that card onto the battlefield under your control. Then that player shuffles.

Acquire

Search target opponent's library for an artifact card and put that card onto the battlefield under your control. Then that player shuffles.

Storm Herd

Create X 1/1 white Pegasus creature tokens with flying, where X is your life total.

Telemin Performance

Target opponent reveals cards from the top of their library until they reveal a creature card. That player puts all noncreature cards revealed this way into their graveyard, then you put the creature card onto the battlefield under your control.

Bribery

Search target opponent's library for a creature card and put that card onto the battlefield under your control. Then that player shuffles.