Argivian Phalanx

This spell costs {1} less to cast for each creature you control.
Vigilance (Attacking doesn't cause this creature to tap.)

Aven Interrupter

Flash
Flying
When Aven Interrupter enters, exile target spell. It becomes plotted. (Its owner may cast it as a sorcery on a later turn without paying its mana cost.)
Spells your opponents cast from graveyards or from exile cost {2} more to cast.

Another Round

Exile any number of creatures you control, then return them to the battlefield under their owner's control. Then repeat this process X more times.

March of Otherworldly Light

As an additional cost to cast this spell, you may exile any number of white cards from your hand. This spell costs {2} less to cast for each card exiled this way.
Exile target artifact, creature, or enchantment with mana value X or less.

Bladewheel Chariot

Tap two other untapped artifacts you control: Bladewheel Chariot becomes an artifact creature until end of turn.
Crew 1 (Tap any number of creatures you control with total power 1 or more: This Vehicle becomes an artifact creature until end of turn.)

Spring-Loaded Sawblades

Flash
When Spring-Loaded Sawblades enters, it deals 5 damage to target tapped creature an opponent controls.
Craft with artifact {3}{W} ({3}{W}, Exile this artifact, Exile another artifact you control or an artifact card from your graveyard: Return this card transformed under its owner's control. Craft only as a sorcery.)

Spring-Loaded Sawblades // Bladewheel Chariot

Resolute Reinforcements

Flash (You may cast this spell any time you could cast an instant.)
When this creature enters, create a 1/1 white Soldier creature token.

Marketwatch Phantom

Whenever another creature you control with power 2 or less enters, Marketwatch Phantom gains flying until end of turn.

Farewell

Choose one or more —
• Exile all artifacts.
• Exile all creatures.
• Exile all enchantments.
• Exile all graveyards.

Plains

Ironpaw Aspirant

When Ironpaw Aspirant enters, put a +1/+1 counter on target creature.

Cosmium Blast

Cosmium Blast deals 4 damage to target attacking or blocking creature.

Inside Source

When Inside Source enters, create a 2/2 white and blue Detective creature token.
{3}, {T}: Target Detective you control gets +2/+0 and gains vigilance until end of turn. Activate only as a sorcery.

Celebrity Fencer

Alliance — Whenever another creature you control enters, put a +1/+1 counter on Celebrity Fencer.

Citizen's Arrest

When Citizen's Arrest enters, exile target creature or planeswalker an opponent controls until Citizen's Arrest leaves the battlefield.

Neighborhood Guardian

Whenever another creature you control with power 2 or less enters, target creature you control gets +1/+1 until end of turn.

Wanderer's Intervention

Wanderer's Intervention deals 4 damage to target attacking or blocking creature.

On the Job

Creatures you control get +2/+1 until end of turn. Investigate. (Create a Clue token. It's an artifact with "{2}, Sacrifice this artifact: Draw a card.")

Leyline Binding

Flash
Domain — This spell costs {1} less to cast for each basic land type among lands you control.
When Leyline Binding enters, exile target nonland permanent an opponent controls until Leyline Binding leaves the battlefield.

Case of the Gateway Express

When this Case enters, choose target creature you don't control. Each creature you control deals 1 damage to that creature.
To solve — Three or more creatures attacked this turn. (If unsolved, solve at the beginning of your end step.)
Solved — Creatures you control get +1/+0.

Gavony Silversmith

When Gavony Silversmith enters, put a +1/+1 counter on each of up to two target creatures.