You're having a fic! Happy Christmas. :)
First off, thank you so much for writing a story just for me. It means so much to me that you would take time out of your undoubtedly busy schedule to write something for a fandom that I adore, which will delight me beyond all measure. You're wonderful, amazing, and just fantastic!
Generally, I like: rich and realistic characterizations, intense and well-developed worldbuilding, vivid imagery and description, mystery, and a bit of the unexpected. I love it when an author takes the expected (like a character or situation I know), and switches it around so that I become embroiled in the mystery of discovering these characters for a second, third, or fourth time.
I dislike: poor grammar, lack of description, flat characters or worldbuilding, or the expected. Nothing makes me stop reading a story faster than boredom and cliche.
Neverwhere - Neil Gaiman
Neverwhere is the one novel that I read over and over again, and love every time. I adore the world that Gaiman built within London, my favorite city in the entire world. For this fandom, I would really like to read a story about London Below. It can include the characters that we know, but it doesn't have to. I just want to learn more about London, feel that I'm being sucked into its spell, that I'm falling into the history of this city and the way that, over time, an ordinary thing like a city can become part of legend.
Jonathan Strange & Mr Norrell - Susanna Clarke
The great thing about Jonathan Strange & Mr Norrell is partly the pastiche and partly the rich and stunning worldbuilding. Clarke built an amazing world for her characters to live in, and the intricate plot both thrilled and astounded me. In this fandom, I would particularly like to see a story that focuses on how the novel's twists and turns have effected each character, or perhaps an original character has been affected by one of the events of the story. Or a story which expands upon and explores the deep world of magic that Clarke built in the novel.
Cities (Anthropomorphic)
As previously mentioned, London is my favorite city in the entire world. I would love to see a story about her as a person, set contemporarily, with her as a hip/trendy girl. I want to see an exploration of London as a modern woman - the person she's become. I would not at all mind seeing some hints of her past (especially the dark parts), but I would prefer that the focus remain on who London is today, rather than how she became what she is.