Here's a challenge I had a go at tackling: make a two-sided portrait card.
So thoughts are:
A card with competing "while this is in play" effects.
A card that does interesting things when it's on top of your deck.
A card with 2 different duration effects, but you have to decide which one you're using when you play the card.
All of them would need the Stash Wording to establish that it is in fact ok to decide were they go in your shuffle.
So a simple one:
FRONTCraftsmanAction - $4
Gain a card costing up to $4
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When shuffling this, you may look through your remaining deck, and may put this anywhere in the shuffled cards.
REARRetireeAction/Duration - $4
For the rest of the game, +1 Action at the start of each of your turns (This remains in play)
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When shuffling this, you may look through your remaining deck, and may put this anywhere in the shuffled cards.