You wouldn't know if someone had been matched on that story before you wrote a treat--sometimes people do wind up writing the same prompt as someone else. Once I was sure I was doing that, but it turned out the person had been matched on something completely different.
When sign-ups open they'll be a whole list of available fandoms that you can choose from, so you sound like you have the right idea looking for ones you can offer. Some of them are even for commercials or songs so you can check them out quickly.
What you can also see as people start signing up is which things have been requested and offered. (Though it doesn't show up on the list until at least one person has requested it.) I think last year, looking at that list, I figured out there was something like 5 requests I might have been matched with or something. And I always wind up being assigned to the one I never thought I'd match on somehow!
no subject
When sign-ups open they'll be a whole list of available fandoms that you can choose from, so you sound like you have the right idea looking for ones you can offer. Some of them are even for commercials or songs so you can check them out quickly.
What you can also see as people start signing up is which things have been requested and offered. (Though it doesn't show up on the list until at least one person has requested it.) I think last year, looking at that list, I figured out there was something like 5 requests I might have been matched with or something. And I always wind up being assigned to the one I never thought I'd match on somehow!