I'm pretty happy with CotO, but I thought this 
image, as posted by Zanz, had some promise for an event.
Here's my take
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Level 1:
Level 2:
Level 3:
Deck-building
Your card-pool is limited to cards of unlocked elements and Other.
Decks can be changed at any time.
Your mark will always be the element you picked in the beginning of the event. No changing.
Battling
People of the same number of unlocked elements will fight each other. Ties will be decided with RNG.
The first person to win twice wins each duel.
Keys/Locks?
#1. Pick a Level 1 element. This will be your mark throughout the whole event.
#2. After each round, you gain 1 "key" of your element. You start the event with 2 keys. If you win a match, then you must steal 1 key of your choice from the opponent. Note that you can always steal the key of the opponent's mark, even if he has 0 of them. He will just not receive it the next round.
#3. Keys can be combined to make new keys, following the equations above. For example, 2 fire keys can be combined to make 1 water key, 1 fire key and 1 earth key can be combined to make 1 gravity key, etc. Note that the keys used will be destroyed.
#4. Keys can be used to unlock "locks." There are 12 locks in all, one for each element. You can only build decks with elements of unlocked locks and other. Locks are affected following the equations above. For example, 1 water key will toggle the water and fire locks, 1 gravity key will toggle the gravity, fire, and earth locks, and 1 darkness key will toggle the darkness and light locks. You start the event with your element's lock unlocked and the 11 other locks locked.
How do you win?
You win by having at least 6 locks unlocked.
Scenario: Alice chooses fire, Bob chooses water.
Alice:
Keys:
x1
Locks:
Bob:
Keys:
x1
Locks:
Alice uses her fire key.
Alice:
Keys: 0
Locks:
Bob:
Keys:
x1
Locks:
Both Alice and Bob can only use Water and Other cards. They fight, Alice using a steam machine deck and Bob using an ice bolt stall. Bob wins. The round ends, and Bob must steal a fire key from Alice, as she doesn't have any keys. Each player gets 1 key of their mark.
Alice:
Keys: 0
Locks:
Bob:
Keys:
x2
x1
Locks:
