First of all, welcome. Thank you so much for writing for me; this is my fifth Yuletide, and it's always one of the highlights of my holiday season. The simple fact you love at least one of the following four fandoms enough to offer to write it for a complete stranger fills me with glee. Therefore, take as a given that anything you write for me will be delightful by dint of existing.
That said, if you'd like some more thoughts or guidance as to my tastes (which I'll assume you do, if you're reading this), read on. I'm generally pretty easy to please genre-wise; angst/fluff/smut/gen whathaveyou are all equally fine. I am not the biggest fan of massive AUs (i.e. radical changes of setting, major changes in a character's canon backstory/gender/etc.) in fanfic as a rule. I'm hard to squick, though; as long as it serves the story, both explicit sex and explicit violence are fine. (That said, some of my favorite fic has also been G-rated, so don't feel I'm pushing you that way.) My favorite fics tend to be plotty rather than meditative, but reveal character through how they act and react to what's going on. That doesn't mean things have to explode (though explosions are fun), but I enjoy seeing characters pushed outside their comfort zones. If it's helpful to you, please feel free to yuletide!stalk my fiction journal, even if it's been a bit quiet recently. I've also unlocked some past yuletide fic recs here, if you'd like to take a look.
Thank you, thank you! For your reference, or for pinch hitters and such, my original notes by fandom are below.
Gentleman Bastard Sequence - Scott Lynch; Any
I am so thrilled to have someone writing in this fandom, that I would be pleased with most anything. If you'd like some direction, Locke and Jean are my platonic OTP, so friendship fic would never go amiss. I also really enjoyed the bits of Nazca and Locke's friendship we got to see, and would love to explore more of that. Bastard hijinks at any point are also great.
Arcadia - Stoppard; Septimus Hodge
Septimus fascinates me, and I would love any sort of fic filling in backstory or projecting past the end of the play. Him and Thomasina are great; cameos by Lord Byron would also not go amiss. Shippy fic is fine, but in no way required.
Downton Abbey; Any
Downton Abbey is a show in which I care about nearly all of the characters equally. Anything about the Crowley sisters, together or separately, would delight me. Bates' backstory would also be an intriguing area to explore. Lord Grantham and Cora's courtship would be intriguing, or more about Sir Richard's background. I think the richness of the relationships between characters is one of the show's strengths, so more than a single focus is completely appropriate.
Le nozze di Figaro | The Marriage of Figaro - Mozart/Da Ponte; Figaro, Susanna
If you're familiar with the source plays, please feel free to use them for additional or alternative inspiration, but if you prefer to stick to opera-verse, that's also fine. The dynamics between the two main couples are endlessly fascinating to me, so feel free to use (or not) the Count and Countess as well. I'd love to know how Susanna and Figaro's courtship went, or what they're up to while offstage during the opera, or how their life after goes. Smut is okay, but not a requirement.