Those are great! I just edited them in.

Should the Batter one be in quotes or not? If it's a general quote (say spoken by an invisible "narrator" of some sorts), I think not - but if it's spoken by the depicted character then it's fine if it's in quotes. In this case the character is the elemental treefolk thing so I'm not so sure.

Play Design Team -
Creature
As ~ enters the battlefield, choose power or toughness, choose a number, and choose more or less.
Creature spells with the chosen stat equal to the chosen number cost more/less to cast.
2/2

Soulbound Demon -
Creature - Demon
Flying
When ~ dies, you lose the game.
6/6


Voluntary Sacrifice -
Sorcery
Any player may have you create a 5/5 black Demon creature token with flying. If a player does, that player loses the game.

Priest of Cosi -
Creature - Human Cleric
Flash
When ~ enters the battlefield, if it’s not your turn, add .
2/1

Stolen Vigor -
Enchantment
When ~ enters the battlefield, if target opponent has more life than you, gain life equal to the difference until ~ leaves the battlefield.

Pre-approval -
Instant
Create a Treasure token.

Derpy Horse -
Creature
Reverse battle cry (Whenever this creature attacks, creatures defending player controls get -1/-0 until end of turn)
0/1

Proliferate —
Instant (C)
Proliferate.

Convert —
Sorcery (U)
Cleave
Gain control of target creature [until end of turn. Untap it. It gains haste].

Good, but I think not being able to remove it (like Mind Control) should make the blue side cost at least 1 more

I think Control Magic has the more recent print than Mind Control, so 5 is probably fine.

Not in a modern-legal set

As shown by Merfolk Trickster you don't need to define p/t here. This could alternatively enchant nonland permanents and not use the "except mana abilities" clause.

At first I had this at and hybrid white-blue was a consideration but now I'm thinking this cost feels the most appropriate.

Good with Death's Shadow

Sounds cute. Get down to 12 or less life, drop shadow, slap this on it, then celebrate for a second for your conditional 13/13 two-card combo before it gets hit by some Fatal Push. I guess it would be worth considering whether currently existing death's shadow deck shells would be willing to splash white for this (as an off-shoot backup surprise strategy of sorts) but somehow I doubt it.

Looking at scryfall with search conditions "mana value <= 2", sort by power descending, there surprisingly don't seem that many lucrative targets for this to be used in that manner.

* https://scryfall.com/search?q=type%3Acr ... sc&as=grid

Disposable Hero -
Creature - Human Soldier
Vigilance
Whenever ~ attacks, it gets +0/+2 until end of turn.
He tests the enemy’s resolve and the quality of the blacksmith’s armor.
3/1


Skeletal Spider -
Creature - Spider Skeleton
Sacrifice a creature: ~ gains reach until end of turn.
2/4

Vigilant Necromancer -
Creature - Human Wizard
Whenever ~ attacks, create two 2/2 black Zombie creature tokens with “At the beginning of your upkeep, sacrifice this creature.”
4/4

Conspicuous Ninja -
Creature - Ninja
~ gets -1/-1 for each other creature you control.
5/5


Mogg Squad doesn’t count, that’s global

Wish I saw Tabula Rasa in time to nominate it

Possibly a dumb cycle taking up space in packs for no reason

Land
: Add .
Cycling

Land
: Add .
Cycling

etc.