I put quite a bit in my sign-up, and I hope I gave you enough characters, pairings and friendship combinations to choose from. Here's more about the pairings and friendships I chose to hopefully help you write. Thank you, Mystery Writer, for writing a story for me!
Also, I came down kind of hard against AU stories in my sign-up, and I'd like to dial that back a bit. I'm aware a lot of people love them and love to write them. I included that because it's hard to explain which AUs I do like. And there are always exceptions. For instance, I like the characters' relationships and dynamic to be close to the same. Say it's a Sterek story, then Stiles and Derek might meet under wildly different circumstances, but if Scott's there, I want him to be Stiles' best friend in whatever different lives they're leading. Don't make Scott Derek's toddler son or his spirit guide or something--that's the kind of thing that's so AU I have a hard time getting around it. I hope that explains my bias a bit better. I have liked and can certainly enjoy AUs that maintain many character connections, so there. ;)
I love the dynamic between Stiles and Derek, in particular how it changed so dramatically in 3a (though something set earlier is fine, too). I like seeing them get together romantically (though I believe it would not be a hearts and flowers and love you baby kind of thing, not saccharine, definitely), and I also like seeing them as allies and friends. A story with one character or the other, like a character study of him just sort of assessing the relationship (friendly or more) with the other would also be awesome. Derek has many issues, so there's no shortage of things to write about if you just want a Derek story. If you'd rather write just Stiles, then I'd be pretty happy with almost anything that wasn't an entire story about the loss of his mom or about his mother. I don't mind a mention of her, she's pretty important to who he is, but I'd rather that not be the focus.
(Eta: I never thought this was something I'd feel the need to put in a letter, but there's a lot of fic like this out there--I don't read girl!Stiles or girl!Derek stories. Sorry. It seems like the list of what I don't want in my sign-up is long, but there's a huge amount of diversity in the TW stories (which is awesome!) and so I listed kind of everything I'd ever run across that I'm not much into.)
I love Peter Hale. He's so manipulative and selfish and bad, but not totally. He has his magnanimous moments (though few), and I'd like to believe there's more good than we think in there. A story with him and Derek interacting (not paired), him and Stiles interacting (not paired) or the three of them in a sass-off would be brilliant. I'm all for Peter and sexual tension with Stiles, or almost anybody really, but I'd rather not have a Peter/anybody story. I'm still fascinated that he gave Stiles a choice when he didn't have much hesitation in chomping on other people. Their dynamic is interesting.
I love Scott and Stiles as best friends. Put them together in almost any situation where their friendship shines through, and I'm a happy girl. I do tend to dislike it when Scott as written as stupid. He has a puppy dog quality, and can be a bit clueless at times, but that's part of his charm, IMO. This is also the guy who replaced Argent's medication with wolfsbane and manipulated Derek into biting Argent. He can be clever and deceitful, and isn't the stooge he's written as a whole hell of a lot of the time. I like it when he sees through Stiles, too.
I want to know more about Boyd, in particular with regards to his sister. Just as he was really getting interesting, he was gone. If you want to write a Boyd-centric story, please go for it! Give him the backstory and richness the show didn't! I'm also a fan of Boyd/Erica, and would love anything that shows them getting together. I can't imagine that was something that came too easy, not with Erica, and I'd love to see what others think.
My favorite female character is Melissa McCall. You could write a Melissa-centric story, a Melissa and Scott story, one with Stiles and Melissa interacting, or one with Melissa and the Sheriff. I'm all for Mama McCall/Papa Stilinski as a couple, so if you love that, go for it. I didn't list Isaac as a character, mostly because I'd rather not have an Isaac-centered story, but if you want to write a story about Scott, Melissa and Isaac in a family-feels sort of way, that would be awesome.
I'd also be happy for a story with Scott, Allison and Isaac in some capacity, whether as three friends (and she can be with Scott or Isaac romantically) or as more. Watching them bond in a polyamorous way would be interesting, and I think the dynamic's really there. And honestly, if the pairing of your heart is Scallison or Scisaac and you'd really rather write a story centered on one of those, please do!
I'm also a big fan of Sheriff Stilinski softening toward Derek (and having any interaction with him at all that doesn't involve Derek in handcuffs), especially now that he's up to speed on the Beacon Hills werewolf hijinks. Derek has made so many mistakes and been punished so much by himself and others. I love it when people show him the compassion and concern he doesn't spare for himself, and that so rarely happens on the show. Damn, even his little sister was awful to him until he saved her life. I'm fond of Stiles being the one to surprise him by actually caring, and I think the Sheriff is a good man who would also eventually see through Derek's walls. I don't want them paired up romantically, though.
Finally, Alan Deaton. I don't entirely trust him or his motives, and have a sort of love/hate relationship with the character, which makes him really interesting. I like his father-figure-ness with Scott, but also want to punch him for his lack of such with Derek. Explain him. Do something with him that's more interesting than the show has so far, and I'll be thrilled.
So those are my takes on the characters and their relationships. If you want to write something during an already-aired season, that's fine. I also don't mind post-3a if you want to put your own spin on what might happen. I'm not really fond of any of the Argents except Allison (okay, Chris is hot, but I have serious issues with him as a person), or any of the Alpha pack, so I'd rather not have them figure in. Any characters I haven't listed are better off left out aside from passing mention, but I hope I gave you enough to work with.
Thank you, thank you, thank you! I'm looking forward to reading what you come up with!