Krark, the Thumbless

Whenever you cast an instant or sorcery spell, flip a coin. If you lose the flip, return that spell to its owner's hand. If you win the flip, copy that spell, and you may choose new targets for the copy.
Partner (You can have two commanders if both have partner.)

Forest

Steel Squirrel

Whenever you roll a 5 or higher on a die, Steel Squirrel gets +X/+X until end of turn, where X is the result.
{6}: Roll a six-sided die.

Strategy, Schmategy

Roll a six-sided die. Strategy, Schmategy has the indicated effect.
1 — Do nothing.
2 — Destroy all artifacts.
3 — Destroy all lands.
4 — Strategy, Schmategy deals 3 damage to each creature and each player.
5 — Each player discards their hand and draws seven cards.
6 — Repeat this process two more times.

Inhumaniac

At the beginning of your upkeep, roll a six-sided die. On a 3 or 4, put a +1/+1 counter on Inhumaniac. On a 5 or higher, put two +1/+1 counters on it. On a 1, remove all +1/+1 counters from Inhumaniac.

Growth Spurt

Roll a six-sided die. Target creature gets +X/+X until end of turn, where X is the result.

Molten Birth

Create two 1/1 red Elemental creature tokens, then flip a coin. If you win the flip, return Molten Birth to its owner's hand.

Pippa, Duchess of Dice

{2}{G}, {T}: Roll a six-sided die. It becomes a green Die creature token with power and toughness each equal to its result.
{2}{U}, {T}: Reroll any die. (Activate only any time it makes sense.)

Mana Screw

{1}: Flip a coin. If you win the flip, add {C}{C}. Activate only as an instant.

Rakdos Carnarium

Rakdos Carnarium enters tapped.
When Rakdos Carnarium enters, return a land you control to its owner's hand.
{T}: Add {B}{R}.

Myrkul's Edict

Roll a d20.
1—9 | Choose an opponent. That player sacrifices a creature of their choice.
10—19 | Each opponent sacrifices a creature of their choice.
20 | Each opponent sacrifices a creature with the greatest power among creatures that player controls.

Hoarding Ogre

Whenever Hoarding Ogre attacks, roll a d20.
1—9 | Create a Treasure token. (It's an artifact with "{T}, Sacrifice this artifact: Add one mana of any color.")
10—19 | Create two Treasure tokens.
20 | Create three Treasure tokens.

Krark's Other Thumb

If you would roll a die, instead roll two of those dice and ignore one of those results.

Mountain

Tavern Scoundrel

Whenever you win a coin flip, create two Treasure tokens. (They're artifacts with "{T}, Sacrifice this artifact: Add one mana of any color.")
{1}, {T}, Sacrifice another permanent: Flip a coin.

Swamp

Six-Sided Die

Choose target creature. Roll a six-sided die.
1 — It has base toughness 1 until end of turn.
2 — Put two -1/-1 counters on it.
3 — Six-Sided Die deals 3 damage to it and you gain 3 life.
4 — It gets -4/-4 until end of turn.
5 — Destroy it.
6 — Exile it.

Sorcerer's Strongbox

{2}, {T}: Flip a coin. If you win the flip, sacrifice Sorcerer's Strongbox and draw three cards.

Barbarian Class

(Gain the next level as a sorcery to add its ability.)
If you would roll one or more dice, instead roll that many dice plus one and ignore the lowest roll.
{1}{R}: Level 2
Whenever you roll one or more dice, target creature you control gets +2/+0 and gains menace until end of turn.
{2}{R}: Level 3
Creatures you control have haste.

Golgari Rot Farm

Golgari Rot Farm enters tapped.
When Golgari Rot Farm enters, return a land you control to its owner's hand.
{T}: Add {B}{G}.

The Space Family Goblinson

The Space Family Goblinson has trample as long as you've rolled three or more dice this turn.
Whenever you roll a die, put a +1/+1 counter on The Space Family Goblinson.

Lobe Lobber

Equipped creature has "{T}: This creature deals 1 damage to target player or planeswalker. Roll a six-sided die. On a 5 or higher, untap it."
Equip {2}

Jumbo Imp

Flying
As Jumbo Imp enters, roll a six-sided die. Jumbo Imp enters with a number of +1/+1 counters on it equal to the result.
At the beginning of your upkeep, roll a six-sided die and put a number of +1/+1 counters on Jumbo Imp equal to the result.
At the beginning of your end step, roll a six-sided die and remove a number of +1/+1 counters from Jumbo Imp equal to the result.

Sylvan Shepherd

Vigilance
Whenever Sylvan Shepherd attacks, roll a d20.
1—9 | You gain 1 life.
10—19 | You gain 2 life.
20 | You gain 5 life.

As Luck Would Have It

Hexproof
Whenever you roll a die, put a number of luck counters on As Luck Would Have It equal to the result. Then if there are 100 or more luck counters on As Luck Would Have It, you win the game. (Count both rolls if you reroll a die.)

Squirrel-Powered Scheme

Increase the result of each die you roll by 2.

Wyll, Blade of Frontiers

If you would roll one or more dice, instead roll that many dice plus one and ignore the lowest roll.
Whenever you roll one or more dice, put a +1/+1 counter on Wyll, Blade of Frontiers.
Choose a Background (You can have a Background as a second commander.)

Gruul Turf

Gruul Turf enters tapped.
When Gruul Turf enters, return a land you control to its owner's hand.
{T}: Add {R}{G}.