They were removed from the game years ago, but were added back with Temporal Forces
Several cards fall into this range, such as PSA 10 Lugia Neo Genesis or high-grade Japanese promo card from the 2000s
Bonus points if youre synergizing with treasures or if you have something spicy to do with the cards that you discard
IIRC in Yugioh there is only a certain number of Blue-Eyes (from the show?), so you could put a rule on the card 'there may only be X Blue-Eyes on the battlefield at any time', Star Wars CCG used to have permanents that you could have 2 or 3 copies of in play, in addition to stuff you could only have 1 of, the concept would work fine here too, only this could care about both players' boards
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