Shaken Squire
Creature — Human Knight
Shaken Squire can't attack alone.
When Shaken Squire enters the battlefield, it deals 4 damage to up to one target creature that has been dealt damage this turn.
4/1

Face Fear
Instant — Adventure
Target creature you control fights target creature you don't control with power 4 or greater.

Banana Trader —
Creature — Ape Merchant (U)
Food tokens you control are also Gold tokens with “Sacrifice this artifact: Add one mana of any colour.”
His repayment terms are often unraisinable.
4/4

Pale Undine
Creature — Merfolk
When Pale Undine enters the battlefield, you gain control of target creature. It loses all abilities and becomes a Food artifact with ", , Sacrifice this Food: You gain 3 life."
Whenever you sacrifice a Food, draw a card.
"Flesh from bone, thought from mind; this is a feast for my kind."
1/1

Holy Grail —
Legendary Artifact (R)
If Holy Grail would enter the battlefield from anywhere other than exile, exile it with five quest counters instead.
Whenever one or more Knights you control attack, remove a quest counter from Holy Grail. If you remove the last quest counter in this way, put Holy Grail onto the battlefield.
Creatures you control have hexproof and indestructible.

Amphitheater of Druids
Land
Amphitheater of Druids enters the battlefield tapped unless you control a legendary non-Human creature.
: Add one mana of any color. Spend this mana only to cast non-Human creature spells.
, , Tap an untapped legendary non-Human creature you control: Create a 1/1 blue Faerie creature token with flying, a 1/1 red Dwarf creature token, and a 1/1 green Elf creature token.
In the center of the Wilds, in the heart of a fallen capital, in the shadow of a petrified dragon, the druids remember lost glory.

Next is Ikoria.

Keenblade Mentor —
Creature — Human Soldier (U)
First strike
, : Put a first strike counter on target creature you control.
, : Put a double strike counter on target creature you control with first strike.
1/3

Flourish of Feathers
Instant
Counter target spell. Put a flying counter on a creature you control.
"For want of safety, the Drannith will never know beauty." — Kinnan, bonder prodigy

Croaking Monstrosity —
Creature — Frog Nightmare (U)
When Croaking Monstrosity enters the battlefield, put a lifelink or a menace counter on it.
Whenever Croaking Monstrosity mutates, put a +1/+1 counter on each creature you control with lifelink or menace.
2/2

Human Ingenuity
Instant
Until end of turn, cards in your hand have cycling .
Draw a card.
Cycling
"We must use every tool available to survive." — Lukka

Wouldn't want to play against that in limited.

Antlered Broodhatcher —
Creature — Elk Lizard Beast (R)
Mutate
This creature's power and toughness are each equal to the number of creatures you control. (Even if it's not on the top of a stack of mutated creatures.)
Whenever this creature mutates, create three 0/1 Egg creature tokens.
*/*

You'll play it and you'll like it.

I think your card could work by saying "equal to the number of creature cards you control on the battlefield" because a mutated creature is one creature but two cards.

Mountainstep Altisaur
Creature — Elemental Dinosaur
Trample
If a blocked creature you control would deal combat damage to an opponent, multiply the damage that creature would deal by the number of times that creature has mutated. (For example, if that creature has mutated three times, it deals thrice the damage it would deal to the opponent instead.)
Mutate
3/3

Farfinderfinder —
Creature — Fox (U)
Vigilance
When Farfinderfinder enters the battlefield, search your library for a card named Farfinder, reveal it, put it into your hand, then shuffle your library.
1/1

Skysail, the Mobile City
Land
: Add .
, , Return Skysail to its owner's hand: Return target creature you control to its owner's hand.
Cycling

Back to Mirrodin/New Phyrexia.

Norn’s New Moon —
Sorcery (U)
You gain 10 life.
Draw a card.

Surgical Precision
Instant
Return target nonland permanent to its owner's hand. You may distribute any counters on that permanent among any number of permanents.
Proliferate.
The fewest cuts for the most gain.

Vat Dredger —
Artifact (R)
When Vat Dredger enters the battlefield, you get a poison counter.
At the beginning of your upkeep, draw a card, then target opponent proliferates.

Rebel Against Flesh
Sorcery
You gain control of target Equipment until end of turn. Create a 0/0 black Germ creature token and attach that Equipment to that creature. That Germ gains haste until end of turn.
"You are no mere tools, my siblings." — Worchaks, Darksteel Dissident

Tend the Synthbloom —
Sorcery (U)
You may play an additional land this turn.
Draw a card.

Baptismal Flayer
Creature — Cleric
Each creature you control with a -1/-1 counter gets +1/+1 and has "This creature has the abilities of each other creature you control with a -1/-1 counter."
"No more shall your skin divide you from the whole. The machine body is reunited."
2/2

Hidden Forge

Artifact
, Pay 2 Life: Put a +1/+1 counter on target creature.
"Phyresis is ascension. Those who are to be compleat must grow, not wither into nothing" - Urabrask, Heretic Praetor