I had the idea that players should be able to use challenge they have finished as a one-time card (of weak or strong nature depending on how it was finished) later in the game to play against another challenge.

For example, I finish a challenge with a strong outcomes, which then grants me a "strong" card with the name of the challenge.

For the sake of balance, I'd also suggest that the narrator would have to mark challenges specifically to be able to played after they're finished. That would prevent over-use of that feature.

Playing a finished challenge as a card would allow players to not only control the outcome of that challenge, but use their (narrative) success to built out the story further.

Having a Character Challenge in the form of an Argument, for example, and finishing it strong, the player can later use that win (in the form as the challenge card (strong) to play against another, fitting challenge (Hey, I beat that NPC in an argument, so I play that as a card against the challenge to gain respect from his friends).

Optionally, instead of giving the won challenge directly to the player, the narrator could put down the card for pickup in a following scene.