Assuming no optional details either, if it was my assignment I would write them a canon-typical gen adventure/casefic/wacky comedy situation as appropriate. Fortunately I'm always happy writing that sort of thing so it wouldn't be a hardship, though it would be frustrating if it was a fandom I'd written several times before where I was hoping for someone else to provide new ideas.

I wouldn't treat a blank signup, though; it would feel a bit too much like making a shameless grab for Yuletide eyeballs on a fic I could have written any time rather than a tailored gift, plus while I'm not generally all that fussed about who most "deserves" treats (I usually have too few options I can write to be that discerning) I'd at least prefer to write for someone invested enough to go to the bare minimum effort of writing one-line prompts, even if they're just, "I would love anything in this fandom!"