Firstly, thank you very much for offering one of my fandoms of choice.
General thoughts:
What I love: Moments of solidarity, people working together against big odds, glimpses of everyday life, and people showing they care about each other. I'd prefer a happy ending for yuletide; open ended or ambiguous is fine, as long as there is a possibility it could all turn out well.
What I don't like: anything dealing with neglect of elderly people or cruelty towards them. Unlikely to come up, I know, but it isn't something I can cope with very easily. The House of the Old in the Giver is fine though - the people are happy and comfortable. Nothing sexually explicit please.
Marlows
Pomona Todd has such and unpromising start, bracketed with Marie Dobson as irritating and unlikable, but she shows she is a lot more than her mother. Interaction with her schoolfellows in or out of school, or even a cameo in a wider tale would be nice to read. One of my favourite scenes is the way the form handles Marie Dobson's death, the squirmy embarrasment of not-feeling-as-expected is captured brilliantly. I'd be happy to read something set during or after the books, whatever inspires you.
Lord Peter Wimsey
After enjoying Clouds of Witness, I was quite disappointed that Mary settled down to married life and appeared only briefly in the narrative. I'd like to read something when she was at school, interactions with her mother, or simply a glimpse of her life. I'd prefer something pre-canon, but if this doesn't inspire, I would enjoy anything to round out Mary. The Dowager Duchess is rather magnificent, and I would love to read about her interactions with her daughter.
The Giver
I first read the Giver when I was about twelve and I remember finishing it on a train and spending the remainder of the journey unpicking it in my mind. I have a weakness for dystopian societies, and I would love to read about the people who live in the community. I'd love to read about life before Jonas became the Receiver - family life when he was small, his parents meeting, a visit from another community - but don't be restricted by this. Let your imagination flow!