I'm less experienced with that problem! I keep a list all year long, and as a result am always overburdened with possibilities.

But my suggestion would be to handle it the same way. Start with the full list and cut out everything you don't know. Look through the much shorter list for things that make you think, "Oh, I hope somebody writes -" Those are your fandoms! And if you have no idea what optional details to offer, or what characters to request - well, the traditional set (romance, backstory, future fic, canon-like, and worldbuilding) might serve you well there.

Or, if you really don't want a story but still want to write one, you can just not sign up. Go through the Dear Author letter spreadsheet or the pinch-hitter list, pick out some prompts you like, write to them, and post them to the recipient. You will have done a good deed! And written something fun!

Either way, have a happy Yuletide. *g*