Not unlike other narrative driven games (as opposed to mechanic driven ones), it might be interesting to look into pacing the outcomes of challenges by the total "value" of cards played (adding up all strong cards, and deducting all weak cards).

Then have the outcome be based on the amount of strong/weak points.
Good, better, best--for strong outcomes. Bad, worse, worst--for weak outcomes.

This idea came to me after playing a six-point challenge. I did the last move, and by that point we were into a weak outcome with three weak and two neutral cards played. Narrative-wise, there was no room for my neutral card (subplot), and it actually would be in character the most to play my strong card. Unfortunately, I felt as if I've wasted a strong card just to finish this challenge.
Would there have been a margin of success, I would have negated the worst possible outcome by reducing the tally from -3 (three bad cards) to -2 (two bad cards, one good card).

I think players can do that anyways if everyone is willing to keep track themselves of how many strong vs weak cards are played.

Though, I would like to see the game giving a little reminder about that when the last card is being played.

For example: "After this move, the challenge will be completed with the best possible outcome." "After this move, the challenge will be completed with a moderate weak outcome."

Something along those lines.