Reincaranation: Target Legendary spell can be your commander.

Monstruosity

Heroic

Mentor

Bestow

Inspired

Bloodrush

Historic

Adapt

Proliferate

Sacrifice

Addendum

Surveil

Spectacle

Riot

Convoke

Afterlife

Jump-start

Create X tokens

Dredge

Mill (self or others)

Delve

Undergrowth

Enchantment Creatures

Constellation

Aura

Equipment

Assist

Dethrone

Will of the council (Vote cards)

Friend or foe

Partner (non-legendaries)

Parley

Monarch

Council's Dilemma (Vote cards)

Tapped lands(Enters the battlefield tapped, taps for X or Y, including guildgates)à

Whenever you cast your second spell

Whenever you cast a non-creature spell

Whenever you cast an instant of sorcery

Whenever you draw your second card

Whenever a creature with power 2 or less

Whenever a creature with power 4-5-6 or more

Card that cares about the same card entering the battlefield (Charmed Stray, Persistent petitioner, etc.)

Power Overwhelming: Add a color to your commander

An Offer You can't Refuse: Reduce the cost of your commander by 1 colorless

Partners in crime: 2 target 2-color legendaries gain the Commander Partner keyword

Long live the king!: When you cast your commander for the first time, you become the monarch.

Ignite your spark! : Target Planeswalker can be your commander.

Alternate win conditions as hidden agendas:
Hail to the king:
You can reveal this card at the begining of your upkeep. When you do, you become the monarch. If you remain the monarch for 2 turns, you win the game.

Master of puppet:
You can reveal this card at the begining of your upkeep. When you do, you gain control of target non-land permanent an opponent control until end of turn. If you manage to kill a player by using the resources of another player (steal creature, redirect spell, redirect damage, etc.), you win the game.

No Man Can Kill Me:
You can reveal this card at the begining of your upkeep. As long as No Man Can Kill Me is revealed, you may spend mana as though if it were mana of any type to cast your commander. Every time you cast your commander, put an experience counter on No Man Can Kill me. When you have 5 experience counter, you win the game.

The Black Death: 
You can reveal this card at the begining of your upkeep. As long as The Black Death is revealed, your commander gains infect. If an opponent dies while having no poison counters, you lose the game.

There is No Spoon:
You can reveal this card at the begining of your upkeep. As long as There Is No Spoon is revealed, whenever a card would be put into exile, put it under There Is No Spoon instead. If there are 100 or more cards exiled under There Is No Spoon, you win the game.

Are You Not Entertained?: 
You can reveal this card at the begining of your upkeep. As long a Are You Not Enternained? Your commander gains Deathtouch and whenever a creature dies to damage dealt by your commander, put a counter on Are You Not Entertained?. If there are 10 or more counters on Are You Not Entertained, you win the game.