I really like the idea of some sort of new goal/reward being established by the game. Especially if the reward was a random card (chosen from all available cards, or maybe excluding pillars). 7 oracle spins per week at (is it 1/12 chance?) would still be the most common way to win Nymphs, but people are always clamoring for another way, and completing a quest for a single random card would satisfy that while making them barely more common, if at all.
But regardless of the reward, I am extremely in favor of some sort of weekly quest!