Undercity Merchant
Rogue Minion (C)
Combo: Add a Coin to your hand.
3/3

Perfect Poison

Rogue Spell (R)
Give your weapon "Destroy any minion dealt damage with this weapon."

Heartrazor (4)
Rogue Weapon (E)
1/4
When your Hero deals damage with this weapon, give it +1 Attack.
The cutting edge of dagger technology.

Elling Trias, "Cheese Vendor"
Rogue Minion (L)
Stealth. Battlecry: Add a Cheese to your hand.
At the start of your turn, Discover a card with Combo.
1/1

Cheese
Rogue Spell (C)
Restore 4 Health to the enemy hero.

That is a ridiculously strong card. It could stand to lose Stealth and trigger at end of turn.

Sen'jin Spirit Guide (4)
Shaman Minion (C)
3/3
Battlecry: Draw a card.

Forbidden Communion
Shaman Spell (E)
Spend all your mana. Draw that many cards and Overload that amount.

Frugar Arcsmasher
Shaman Minion (L)
Battlecry: Draw a card for each friendly totem.
7/7

Wretched Researcher
Warlock Minion (C)
If you draw this by using your Hero Power, it gets +1/+1.
Whenever you use your Hero Power while this is in your hand, it gets +1/+1.
1/1

Unholy Contract
Warlock Spell (R)
If your board is full, draw seven cards.

Misery's Advocate
Warlock Minion (E)
Battlecry: The enemy hero's Hero Power becomes 'Draw a card and take 2 damage.'
[Demon]
8/6

Socrethar the Eternal
Warlock Minion (L)
Battlecry: You become IMMORTAL*!
[Mech]
8/8

You get five copies of the following card. They burn away the leftmost cards in your hand to make room.

Sargeras' Debt
Warlock Spell
You are Immune as long as this is in your hand, except you take 3 damage at the start of your turn.

Summon: They have promised us the glory of immortality!
Attack: Join me in death!
Attack Prophet Velen: You are a fool!
Death: The nether! It consumes me!

so an 8/8 that deals you 15 damage per turn?

It makes you IMMORTAL.

(I imagined it as a kind of Ice Block effect for control warlock, combined with discard effects, etc.)

Just 3 of those cards would probably still be balanced.

It was originally just one debt that dealt 10 damage, and then two that dealt 5, then five that dealt 2.

Barrel Roller —
{art: a pandaren using Chen's Wandering Keg ultimate from Heroes of the Storm}
Warrior Minion (C)
Charge
1/4

Forbidden Raider
Warrior Minion (R)
Charge. Battlecry: Spend all your mana. Gain +1 Attack for each mana spent.
[Pirate]
0/3

Really tried to find something to work with Warsong Commander.

Rallying Cry (5)
Warrior Spell (E)
Give all the minions in your hand and deck +2 Attack and Charge. Draw two cards.

Strom'kar, the Warbreaker
Warrior Weapon (L)
Deals double damage to Taunts.
Whenever your hero attacks, friendly minions gain +2 Attack and Charge this turn.
6/2