msem-instigator

When Lost Kitten leaves the battlefield, create a 1/1 white Peasant creature token.

When Irreverent Sinscape enters, destroy up to one other target nonland permanent.
When Irreverent Sinscape dies, exile it. When you do, you may exile any number of cards from your graveyard that have four or more card types among them. If you do, return a permanent card from a graveyard to the battlefield under your control. You may repeat this process any number of times.

Enchant creature
Enchanted creature gets +1/+1 and has flying.
When Skypath Pilgrimage enters, journey twice. (Exile the top two cards of your library face up. Whenever a creature you control deals combat damage to an opponent, put a journeyed card you own into your hand.)

Golden age (If you cast an instant or sorcery spell, activated a nonmana ability, and attacked this turn, enter a renaissance for this game.)
{2}{R}{G}{W}: You may put a creature card from your hand onto the battlefield. It gains haste and vigilance until end of turn. At the beginning of the next end step, return it to your hand unless you're in a renaissance.

Vigilance
Whenever Liege of Asham attacks or blocks, other attacking or blocking creatures you control get +1/+1 until end of turn.

({T}: Add {R} or {W}.)
Spiraling Canyon enters tapped unless you control a basic Mountain or a basic Plains.

Harmony — Search your library for a basic land or Desert card, reveal it, and put it into your hand. Then shuffle.
When Trumpeting Pathmaker enters, harmonize. (Trigger up to one harmony of a permanent you control.)

You draw two cards, lose 2 life, and mill two cards.

Alpha (You may have this creature enter with a +1/+1 counter on it. If you do, it becomes alpha and can only attack alone.)
Hexproof

When Marshcroaker Coaching enters, create three 1/1 blue Frog creature tokens.
Frogs you control have “{T}: Target creature gains flying until end of turn.”

If an enchanted permanent you control would leave the battlefield, instead destroy one of the Auras attached to it, then that permanent phases out. (Treat it and anything attached to it as though they don't exist until its controller's next turn.)

As an additional cost to cast this spell, sacrifice a creature.
Surveil 2, then draw a card. Create a Treasure token and a 1/1 black Bat creature token with flying and “This creature can't block.”

Permanents you control with mana value 1 or less have first strike, haste, and lifelink.

Creatures your opponents control get -2/-0 until end of turn. Scry 2.

{1}, {T}: Exile the top two cards of your library, then put a card exiled with Fate Charm on top of its owner's library.

Whenever Conjured Stalker attacks, you may draw a card. If you do, discard a card.

Whenever Icepeak Phantasm becomes the target of a spell or ability, an opponent may pay {2}. If they do, sacrifice it.

Whenever you draw a card, put a hatch counter on Reefcrown Egg.
{T}, Sacrifice Reefcrown Egg: You may put a creature card with mana value less than or equal to the number of hatch counters on Reefcrown Egg from your hand onto the battlefield.
Cycling {2} ({2}, Discard this card: Draw a card.)

Sunny Dunes enters tapped unless another player controls a land that could produce {R} or {W}.
{T}: Add {R} or {W}.

When Paint by Numbers enters, put up to ten brush counters on it.
Whenever a player casts a instant or sorcery spell with mana value equal to the number of brush counters on Paint by Numbers, remove a brush counter from it and copy that spell. You may choose new targets for the copy. Then sacrifice Paint by Numbers if it has no brush counters.

When Treacherous Warden enters, search your library for a card, put into your graveyard, then shuffle.
Whenever a creature enters from your graveyard, each opponent loses 2 life and you gain 2 life.

Other Humans you control get +1/+1.
Whenever Righteous Chaplain or another Human you control enters, tap target creature an opponent controls.

Flash
When Wormwood's Agent dies, create a Vial token. (It's an artifact with “{1}, Sacrifice this and another permanent: Draw a card. Activate only during your turn.”)

Spirited (When this creature enters, you may activate one of its activated abilities without paying its cost.)
{3}{W}{B}: Create a 0/1 white and black Human Advisor creature token with “When this creature enters, target creature can't attack or block until this creature leaves the battlefield.”

Look at the top two cards of your library. Put one of them into your hand, then you may have any number of players each mill three cards.