Basically, I will be excited by anything you write. I'm not a difficult person to please. I like my fanfiction in gen, het or slash varieties; it can be clean as a whistle or as smutty as you wanna get. I'm fond of humor, family affection, romance based on friendship, wacky adventures, sex in unexpected places, good-natured competition and even the occasional inevitable heartbreak. The only things I would ask you to avoid are humiliation and cruelty. I don't dig them, even in fanfiction.
Also, if you want to have a look at a fandom you don't recognize, the three movies are all on DVD and accessible through Netflix. The book should be available through any bookstore or your local library.
Fandom-specific guidelines, repeated from my signup form:
Three Men and a Baby (Peter/Jack)
This movie reminds me of my childhood, so of course now I want to dirty it up completely. My ideal story would be some kind of Peter/Jack romance, possibly prior to the events of the movie. However, I'd also be really happy with a "how they met" story, or something from during or after the movie showcasing the relationship between these guys, romantic or platonic. You can use Three Men and a Little Lady as canon or ignore it. My one caveat: please don't make Sylvia a bitch.
Troop Beverly Hills (any)
Another childhood favorite. Maybe the girls meet up for a troop reunion, or a wedding? Did they stay friends or have they lost touch with each other? How is Phyllis handling her fifties? Basically anything using these characters will thrill me. Go wild!
An Education (Jenny/Danny/Helen)
I found this movie just fascinating in a lot of ways, but one of the things that struck me the most was that last conversation between Jenny and Danny and Helen. It seemed like they showed a spark of real empathy with Jenny, for all that he's a cad and she's kind of dim. My ideal story would be a spark of a sexier sort, maybe a chance meeting that takes an unexpected turn? If that's not your cup of tea, I'd enjoy any sort of story linking these three together, during the time of the film or after.
A Tree Grows in Brooklyn- Betty Smith (Francie Nolan)
It's 1967 and Francie opens her envelope, the one she sealed fifty years before. Where has her life taken her in between? Did she marry Ben? Did she ever see Lee again? What's her relationship with her mother, with Neeley, with Annie Laurie? What does her future hold? A story about any of these questions would make me very happy.
I hope this is helpful, most excellent Yuletide author, and not too scary.