This spell can't be countered.

You may have Altered Ego enter the battlefield as a copy of any creature on the battlefield, except it enters with X additional +1/+1 counters on it.

{T}: Add one mana of any color in your commander's color identity.

({T}: Add {G} or {U}.)

As Breeding Pool enters the battlefield, you may pay 2 life. If you don't, it enters the battlefield tapped.

You may have Clever Impersonator enter as a copy of any nonland permanent on the battlefield.

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

{T}: Add one mana of any color in your commander's color identity.

Counter target spell.

Search your library for up to two basic land cards, reveal those cards, put one onto the battlefield tapped and the other into your hand, then shuffle.

You may have Dack's Duplicate enter the battlefield as a copy of any creature on the battlefield, except it has haste and dethrone. (Whenever it attacks the player with the most life or tied for most life, put a +1/+1 counter on it.)

When Eternal Witness enters the battlefield, you may return target card from your graveyard to your hand.

Search your library for a Plains, Island, Swamp, or Mountain card, put it onto the battlefield tapped, then shuffle.

({T}: Add {G}.)

Frontier Bivouac enters the battlefield tapped.

{T}: Add {G}, {U}, or {R}.

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

You may have Glasspool Mimic enter the battlefield as a copy of a creature you control, except it's a Shapeshifter Rogue in addition to its other types.

Glasspool Shore enters the battlefield tapped.

{T}: Add {U}.

Draw a card. You may put a land card from your hand onto the battlefield.

({T}: Add {U}.)

({T}: Add {G}, {U}, or {R}.)

Ketria Triome enters the battlefield tapped.

Cycling {3} ({3}, Discard this card: Draw a card.)

Search your library for up to two basic land cards, reveal those cards, put one onto the battlefield tapped and the other into your hand, then shuffle.

You may have Mercurial Pretender enter the battlefield as a copy of a creature you control, except it has "{2}{U}{U}: Return this creature to its owner's hand."

Flying, ward {2}

Whenever another nontoken Dragon enters the battlefield under your control, create a token that's a copy of it, except the token isn't legendary.

You may have Mirror Image enter as a copy of a creature you control.

You may have Mirrorhall Mimic enter the battlefield 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.)

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.

({T}: Add {R}.)

({U/P} can be paid with either {U} or 2 life.)

You may have Phyrexian Metamorph enter the battlefield as a copy of any artifact or creature on the battlefield, except it's an artifact in addition to its other types.

You may have Progenitor Mimic enter the battlefield as a copy of any creature on the battlefield, except it has "At the beginning of your upkeep, if this creature isn't a token, create a token that's a copy of this creature."

Creature spells you control can't be countered.

Nontoken creatures you control have riot. (They enter the battlefield with your choice of a +1/+1 counter or haste.)

Kicker {5} (You may pay an additional {5} as you cast this spell.)

Create a token that's a copy of target creature. If this spell was kicked, create five of those tokens instead.

Flash

Cascade (When you cast this spell, exile cards from the top of your library until you exile a nonland card that costs less. You may cast it without paying its mana cost. Put the exiled cards on the bottom of your library in a random order.)

You may have Sakashima's Protege enter the battlefield as a copy of any permanent that entered the battlefield this turn.

Ninjutsu {1}{U} ({1}{U}, Return an unblocked attacker you control to hand: Put this card onto the battlefield from your hand tapped and attacking.)

You may have Sakashima's Student enter the battlefield as a copy of any creature on the battlefield, except it's a Ninja in addition to its other creature types.

Search your library for up to two Forest cards, put them onto the battlefield, then shuffle.

{T}: Add {C}{C}.

You may have Spark Double enter the battlefield as a copy of a creature or planeswalker you control, except it enters with an additional +1/+1 counter on it if it's a creature, it enters with an additional loyalty counter on it if it's a planeswalker, and it isn't legendary.

({T}: Add {U} or {R}.)

As Steam Vents enters the battlefield, you may pay 2 life. If you don't, it enters the battlefield tapped.

({T}: Add {R} or {G}.)

As Stomping Ground enters the battlefield, you may pay 2 life. If you don't, it enters the battlefield tapped.

Flash

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

Creatures you control have haste.

Whenever a creature with power 4 or greater enters the battlefield under your control, you may draw a card.

Trample

Whenever Thrakkus the Butcher attacks, double the power of each Dragon you control until end of turn.

You may have Undercover Operative enter the battlefield 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.)