Storage
$?? - Action/Reaction
(Insert something here)
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When you would gain a card, you may put it on your Storage mat. Return your storage mat to your deck at the end of the game.
Like an immediate Island. What do you think?
Storage
Something
When you
would gain a card, you may reveal this from your hand. If you do, put the gained card on your Storage mat. Return all cards on your storage mat to your deck at the end of the game.
I don't know if "would gain" is a good idea here. You can't return something that has never been in your deck. And if you gain it at the end of the game, that's kind of weird with cards like Border Village... Personally i don't like putting Storage itself on the mat, but if it doesn't, make it cost enough. Would be at least 4$, i guess, probably more.
Edit: I don't know if it is broken, but it sure is strong. Like a Watchtower you can use to maintain a Golden deck and that you can use for Gardens.