Well of Lost Dreams

Whenever you gain life, you may pay {X}, where X is less than or equal to the amount of life you gained. If you do, draw X cards.

Yannik, Scavenging Sentinel

Partner with Nikara, Lair Scavenger
Vigilance
When Yannik enters, exile another creature you control until Yannik leaves the battlefield. When you do, distribute X +1/+1 counters among any number of target creatures, where X is the exiled creature's power.

Dawn of Hope

Whenever you gain life, you may pay {2}. If you do, draw a card.
{3}{W}: Create a 1/1 white Soldier creature token with lifelink.

Stunt Double

Flash
You may have this creature enter as a copy of any creature on the battlefield.

Laboratory Maniac

If you would draw a card while your library has no cards in it, you win the game instead.

Abdel Adrian, Gorion's Ward

When Abdel Adrian enters, exile any number of other nonland permanents you control until Abdel Adrian leaves the battlefield. Create a 1/1 white Soldier creature token for each permanent exiled this way.
Choose a Background (You can have a Background as a second commander.)

Phantom Steed

Flash
When this creature enters, exile another target creature you control until this creature leaves the battlefield.
Whenever this creature attacks, create a tapped and attacking token that's a copy of the exiled card, except it's an Illusion in addition to its other types. Sacrifice that token at end of combat.

Gigantoplasm

You may have this creature enter as a copy of any creature on the battlefield, except it has "{X}: This creature has base power and toughness X/X."

Luminous Phantom

Flying
Whenever another creature you control leaves the battlefield, you gain 1 life.
If Luminous Phantom would be put into a graveyard from anywhere, exile it instead.

Lunarch Veteran

Whenever another creature you control enters, you gain 1 life.
Disturb {1}{W} (You may cast this card from your graveyard transformed for its disturb cost.)

Lunarch Veteran // Luminous Phantom

Soul of the Harvest

Trample
Whenever another nontoken creature you control enters, you may draw a card.

Wall of Lost Thoughts

Defender (This creature can't attack.)
When this creature enters, target player mills four cards. (They put the top four cards of their library into their graveyard.)

Sage's Row Denizen

Whenever another blue creature you control enters, target player mills two cards.

Champion of Lambholt

Creatures with power less than this creature's power can't block creatures you control.
Whenever another creature you control enters, put a +1/+1 counter on this creature.

Clone

You may have this creature enter as a copy of any creature on the battlefield.

Plains

Island

Ghastly Mimicry

Enchant creature
At the beginning of your upkeep, create a token that's a copy of enchanted creature, except it's a Spirit in addition to its other types.
If Ghastly Mimicry would be put into a graveyard from anywhere, exile it instead.

Mirrorhall Mimic

You may have this creature enter as a copy of any creature on the battlefield, except it's a Spirit in addition to its other types.
Disturb {3}{U}{U} (You may cast this card from your graveyard transformed for its disturb cost.)

Mirrorhall Mimic // Ghastly Mimicry

Daxos, Blessed by the Sun

Daxos's toughness is equal to your devotion to white. (Each {W} in the mana costs of permanents you control counts toward your devotion to white.)
Whenever another creature you control enters or dies, you gain 1 life.

Angel of Glory's Rise

Flying
When this creature enters, exile all Zombies, then return all Human creature cards from your graveyard to the battlefield.

Theoretical Duplication

Whenever a nontoken creature an opponent controls enters this turn, create a token that's a copy of that creature.

Vizier of Many Faces

You may have this creature enter as a copy of any creature on the battlefield, except if this creature was embalmed, the token has no mana cost, it's white, and it's a Zombie in addition to its other types.
Embalm {3}{U}{U}

Elemental Bond

Whenever a creature you control with power 3 or greater enters, draw a card.

Bess, Soul Nourisher

Whenever one or more other creatures you control with base power and toughness 1/1 enter, put a +1/+1 counter on Bess.
Whenever Bess attacks, each other creature you control with base power and toughness 1/1 gets +X/+X until end of turn, where X is the number of +1/+1 counters on Bess.

Heronblade Elite

Vigilance
Whenever another Human you control enters, put a +1/+1 counter on this creature.
{T}: Add X mana of any one color, where X is this creature's power.

Lonis, Cryptozoologist

Whenever another nontoken creature you control enters, investigate.
{T}, Sacrifice X Clues: Target opponent reveals the top X cards of their library. You may put a nonland permanent card with mana value X or less from among them onto the battlefield under your control. That player puts the rest on the bottom of their library in a random order.

Radiant Solar

Flying, lifelink
Whenever this creature or another nontoken creature you control enters, venture into the dungeon. (Enter the first room or advance to the next room.)
{W}, Discard this card: Venture into the dungeon and you gain 3 life.

Kyler, Sigardian Emissary

Whenever another Human you control enters, put a +1/+1 counter on Kyler.
Other Humans you control get +1/+1 for each counter on Kyler.

Glorious Protector

Flash
Flying
When this creature enters, you may exile any number of non-Angel creatures you control until this creature leaves the battlefield.
Foretell {2}{W}

Fiend Hunter

When this creature enters, you may exile another target creature.
When this creature leaves the battlefield, return the exiled card to the battlefield under its owner's control.

Haliya, Guided by Light

Whenever Haliya or another creature or artifact you control enters, you gain 1 life.
At the beginning of your end step, draw a card if you've gained 3 or more life this turn.
Warp {W} (You may cast this card from your hand for its warp cost. Exile this creature at the beginning of the next end step, then you may cast it from exile on a later turn.)

Altered Ego

This spell can't be countered.
You may have this creature enter as a copy of any creature on the battlefield, except it enters with X additional +1/+1 counters on it.

Wormfang Drake

Flying
When this creature enters, sacrifice it unless you exile a creature you control other than this creature.
When this creature leaves the battlefield, return the exiled card to the battlefield under its owner's control.

Lumbering Battlement

Vigilance
When this creature enters, exile any number of other nontoken creatures you control until it leaves the battlefield.
This creature gets +2/+2 for each card exiled with it.

Teferi's Tutelage

When this enchantment enters, draw a card, then discard a card.
Whenever you draw a card, target opponent mills two cards. (They put the top two cards of their library into their graveyard.)

Horizon Chimera

Flash
Flying, trample
Whenever you draw a card, you gain 1 life.

Garruk's Packleader

Whenever another creature you control with power 3 or greater enters, you may draw a card.

Undercover Operative

You may have this creature enter as a copy of any creature on the battlefield, except it enters with a shield counter on it if you control that creature. (If it would be dealt damage or destroyed, remove a shield counter from it instead.)

Forest

Lagrella, the Magpie

When Lagrella enters, exile any number of other target creatures controlled by different players until Lagrella leaves the battlefield. When an exiled card enters under your control this way, put two +1/+1 counters on it.

Restoration Angel

Flash
Flying
When this creature enters, you may exile target non-Angel creature you control, then return that card to the battlefield under your control.

Felidar Guardian

When this creature enters, you may exile another target permanent you control, then return that card to the battlefield under its owner's control.

Sun Titan

Vigilance
Whenever this creature enters or attacks, you may return target permanent card with mana value 3 or less from your graveyard to the battlefield.

Psychosis Crawler

Psychosis Crawler's power and toughness are each equal to the number of cards in your hand.
Whenever you draw a card, each opponent loses 1 life.

Ivy Lane Denizen

Whenever another green creature you control enters, put a +1/+1 counter on target creature.