It seems like having the last player to play a card on a challenge win control of the scene's completion makes it largely a function of who is online and posting at the right time, not the person that made the best contribution to the challenge. I suggest an alternate completion scheme (that perhaps the narrator can choose (on game startup), the 'classic' finishing the challenge or the 'new' best completion. 'Best' would require more work for the narrator, however, as once a challenge was completed they would select the player that made the best contribution to the scene to complete it (then the selected player would complete the challenge with a new post). It feels more story-like to me rather than gamishness.