The Great List of Arena Cards (Current count: 16 arenas)
Blood Guard Colosseum by @EricF
You must use the first card drawn each turn as your combat card. You may reveal an Ace or Joker drawn this way to ignore this requirement for the turn. (On turns without a draw phase, you can play any card).
"Give all you got in the fight times, never give up!" -Drakk Ramhorn
Clock Tower by @Shadow_Night_Black
During the combat reveal, you may discard an Ace to swap your combat card with the top card of your discard pile. (The player with the higher remaining life decides whether to swap first. Decide randomly if there is a tie.)
Geiger's experiments with time have created pockets of temporal distortion throughout his workshop.
Comeback Central by @pengwndude
The first time you fall below 50 Life, fetch a Joker. The first time you fall below 10 life, fetch two Aces. (You may fetch cards from your deck or discard.)
Never count out someone backed into a corner!
Final Destination by @Zqxx
You no longer lose when your life is at 0. Each player's life starts at 0, and damage causes life to increase. (Healing decreases HP. You cannot go below 0 HP.)
If a player is at or above their character's maximum life, they lose if they are damaged with an Ender or Can't Combo move that deals more than 1 damage. (Hitting with an Ender or Can't Combo move that raises the opponent's life to max, or hitting with an Ender in a combo that raises their life to max, does not count.)
If the game ends by time out, the player with lower life wins.
"No EX cards, Zane only, Final Destination." -Some Smash Bros. player, probably
Flagstone Asylum by @Shadow_Night_Black
If you reveal the same combat option (Attack, Throw, Block or Dodge) as you did last turn, discard a card.
It is said that insanity is doing the same thing over and over again and expecting different results.
Fox's Den School by @ThreeHeadedMonkey
If you combat-reveal a Dodge, return it to your hand unless you were thrown.
Those who train to become ninja must be light on their feet, able to dodge even the fastest attacks with ease.
Hot Springs Spa by @ThreeHeadedMonkey
Whenever a player powers up, they retrieve one more Ace than usual. (You can only power up if you would normally be able to do so.)
You'll feel better after a hot bath.
Inverse Arena by @Zqxx
Attacks beat Blocks and Dodges, Blocks and Dodges beat Throws, and Throws beat Attacks. (Jokers still beat Attacks and Throws and lose to Blocks and Dodges.)
You must unlearn everything you've learned.
Liar's Lair by @Zqxx
During the draw phase, each player declares Attack, Throw, Block, or Dodge. If a player combat-reveals the option they declared, they immediately draw a card. (The player with the highest life declares first.)
Trust no one.
Lost Temple by @Zejety
Neither player gets normal draw. Skip the clean up phase of each turn. (Don't draw if your normal attack is blocked or wins combat, and don't discard down to 12 cards at the end of each turn.)
The sight of these ruins feels strangely familiar.
Master's Dojo by @Hobusu (Inspired by some posts by @Leontes)
Remove all Jokers from each player's deck before starting this game.
Learning respect for others is as important as learning to fight honorably.
Murkwood Swamp by @Hobusu
At the end of each turn, each player takes 2 damage.
The swamps of Murkwood sap the life of those who dare enter them.
Pandaland Buffet by @Hobusu
Draw an extra card each draw phase.
Happy martial artists spar more.
Tavern by @Hobusu
During the Combat Reveal, you may discard a card of the same suit as your normal attack to make your opponent vulnerable to mixup normals that turn. (If you hit them with a mixup normal this way, they discard their block card and do not draw a card.)
People fight dirty here, so don't hold back!
Valerie's Studio by @Barrelfish
Starters and Enders are treated as Linkers. (Card abilities that would change the combo type of a card have priority over this.)
"I see the world a bit differently." -Valerie Rose
Vortoss Ruins by @Zqxx
If both players combat-reveal the same type of move, the slower move wins.
Time flows differently here.
Original Post (This is only here for historical reasons)
Considering how fun the Map Cards in Codex and the Casino cards in Pandante are, I thought it would be fun to think about what that would look like for Yomi. The guiding principle behind them seems to be that each card should change the rules in a simple way, but that change should have a significant impact on how you play that round. They're also not necessarily intended to be balanced -- much like the EX characters, trying to argue one of these needs changed because it's unbalanced is Missing the Point, unless leaving it unchanged would break the game. With that in mind, here's some ideas I had (most of which probably aren't simple enough):
Snowy Mountains: At the end of each turn, take 2 damage if you didn't successfully block an attack or Joker.
Tavern: During the Combat Reveal, you may discard a card of the same suit as your normal attack to make your opponent vulnerable to mixup normals this turn.
Master's Dojo: Remove all Jokers from each player's deck this game.
Fox's Den School: You may follow up a successful dodge with a full combo, but face cards and aces cost 1 more combo point each this turn if you do.
Pandalands: Draw an extra card each Draw Phase.
I'm sure there's lots of room for more ideas! I've only started playing Yomi recently, so I don't know the game well enough to see where small changes would make a big impact. That said, please share any ideas you have, whether it's new cards or improvements to these ones.