Was not implying the card is (too) powerful; I don't think it is. I just thought it was great design.

Wood of Shadows
Land
~ enters the battlefield tapped.
Whenever a creature dies, put a +1/+1 counter on ~.
: Add or to your mana pool.
: ~ becomes a 1/1 Green and Black Treefolk Horror creature until end of turn.

Snow-Capped Peak
Land
~ enters the battlefield tapped.
Whenever you cast an instant or sorcery spell, put a +1/+1 counter on ~.
: Add or to your mana pool.
: ~ becomes a 1/1 Blue and Red elemental creature until end of turn.

Algae Bloom
Land
~ enters the battlefield tapped.
Whenever you cast a creature spell, put a +1/+1 counter on ~.
: Add or to your mana pool.
: ~ becomes a 1/1 Blue and Green Ooze creature until end of turn.

Castle Barracks
Land
~ enters the battlefield tapped.
Whenever a creature you control deals combat damage to a player, put a +1/+1 counter on ~.
: Add or to your mana pool.
: ~ becomes a 1/1 Red and White Soldier creature until end of turn.

Haunted Cathedral
Land
~ enters the battlefield tapped.
Whenever you're dealt damage by a source an opponent controls, put a +1/+1 counter on ~.
: Add or to your mana pool.
: ~ becomes a 1/1 Black and White Spirit creature until end of turn.

Some of these feel a bit strong, particularly the U/R one, it is basically Myth Realized without the card slot.

Except that there's a big difference between paying to use it and paying to use it, especially since you're paying it every turn. The standard manland efficiency rule seems to be that the creature should be about what you would get for the activation cost (see the Zendikar cycle). The card I considered for this one was Spellheart Chimera. Sure, this doesn't stay within bolt range forever the way chimera does, but it costs more, doesn't have flying or trample, and is a horrible, HORRIBLE topdeck, so I think it's fine.

More importantly though, myth realized and spellheart chimera are unplayably bad cards, so even if this was strictly better than both of them, I wouldn't be worried.

Some protection shennanigans:

Unwanted Assistance
Instant
Target creature you don't control gains protection from the color of your choice until end of turn.
Draw a card.
Arek the bodyguard would diligently protect his master from poisoning by "testing" all of his food and wine.

Saving Grace
Instant
Split second
Target creature you control gains protection from the color of your choice until end of turn.

Sword and Shield
Instant
Target creature gets +2/+0 and gains protection from the color of your choice until end of turn.

Spectrum Shield
Instant
Target creature you control gains protection from any number of colors until end of turn.

Cursed Blade
Artifact Equipment
Equipped creature gets +3/+3
At the beginning of your upkeep, put a -1/-1 counter on equipped creature.
Equip
It doesn't give its wielder power, it lends it, and with interest.

Lawkeepers' Sanctum
Land
~ enters the battlefield tapped.
: Add to your mana pool.
: Until end of turn, ~ becomes a 1/1 white Soldier creature and gains ", : Target creature can't attack this turn."

Enchanted Woods
Land
~ enters the battlefield tapped.
: Add to your mana pool.
: Until end of turn, ~ becomes a 1/1 green Treefolk creature and gains ", : Target creature gets +1/+1 until end of turn."

Pitch-Black Marsh
Land
~ enters the battlefield tapped.
: Add to your mana pool.
: Until end of turn, ~ becomes a 1/1 black Horror creature and gains ", : Target player loses 1 life."

Fiery Forge
Land
~ enters the battlefield tapped.
: Add to your mana pool.
: Until end of turn, ~ becomes a 1/1 red Elemental creature and gains ", : Target creature can't block this turn."

Bottomless Sea
Land
~ enters the battlefield tapped.
: Add to your mana pool.
: Until end of turn, ~ becomes a 1/1 blue Merfolk creature and gains ", : Scry 1."

Bloodboiler
Creature - Devil
If ~ entered the battlefield this turn, creatures you control have haste.
Suspend 1
2/1

How many turns is it suspended for?

Oops, sorry. It's suspend 1.

Turn Down
Instant
Counter target spell, then fateseal x. That spell's controller reveals the top card of their library, and plays it without paying its mana cost.
And for what?

fateseal is mean

I like the reference, I don't like the one mana counterspell (or one mana emrakul).

Swift-Fist Adept
Creature - Human Monk
Prowess
Whenever you cast a noncreature spell, ~ gains prowess until end of turn.
3/3

Math is hard.

Warden of Decay
Creature - Human Shaman
When ~ enters the battlefield, distribute 3 -1/-1 counters among creatures you control.
6/7

Most of the time that's just gonna be sacrifice a creature

Most of the time, yes. However, sometimes it will be a vanilla 3/4, and sometimes it will be shennanigans.

Alkas' Corsair
Creature - Bird Pirate
Flying
When ~ enters the battlefield, draw a card and you gain 6 life.
2/2

EDIT: oops, forgot flying. This isn't exterminators block, we don't have flightless birds.

No flying?

If it has flying, I like the card but I wonder why it draws you a card.