Take a read of the existing ones, (& review ):
http://original.adult-fanfiction.org/story.php?no=600108028
http://original.adult-fanfiction.org/story.php?no=600108385
http://original.adult-fanfiction.org/story.php?no=600108780
Just a Halloween element and original is the tradition for the Halloween ones. Though, I choose to make mine related to my existing stories in some fashion, so in mine, I’ve done:
The Phone: A “haunted” phone
Sweater: Bullying’s effects (around Halloween)
Bones: Teens in a haunted Indian burial ground on Halloween.
For the other round robin, it’s holiday related, so:
The Phone, Part II: Sequel to the phone, (holiday followup)
Mistletoe Hanging: A teen boy goes to his school’s winter dance
Jump: Helping a veteran on Christmas Eve
Baby Shark: Two boys around Christmas
And the two boys one, I started having a lot of ideas, too many; so I took the single holiday idea, wrote it, while the rest of the plot bunnies became the longer chaptered sequel, Dale’s Game.
If you do as I do, place it into an existing original universe, don’t expect the readers to have read any of your existing works. (As its kinda why I decided to submit in the first place, to hopefully lure some readers in to my other stories.)