That would also mean it would have to be designed almost perfectly to a tee and balanced well enough that it couldn't be broken.
But then again, you do have a point. RP Hosts could find ways to actually create a new TCG in the process and have the applicants learn the game at the same time.
I don't think that's necessary. It's fine to be broken and have the characters/players discover and abuse that as they play along, that's part of the immersion and can really help to set the characters' decks and personalities apart from each other. After a while the story can just ban or do something to mitigate those issues, and let players try to find another thing to abuse. It's not an actual card game so you don't have the same constraints for say, making a banlist.
As for designing individual cards, they'd have to go through a host or co-host first(in such an RP, a co-host can just be someone who's eager about creating cards and likes the game, not necessarily someone who'd have to participate in the RP a lot). if said custom card game has a cost-system then you can have that be the guideline for how much a card of a certain cost can accomplish.
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