...thank you so much for having signed up to write one of these fandoms/pairings! More than anything else, I hope you will enjoy writing at least some of these prompts/ideas. That said:
General likes: humor/snark, long plotty fic, canon pairings, retellings or resonance with myth/folklore/etc., worldbuilding, idealism, playing with or twisting tropes, AUs with subtle callbacks to canon, women being awesome in multiple (not just Strong Female Character, to quote Kate Beaton) ways, romance not being the sole concern or happiness in characters' lives. I also adore backstory or behind-the-scenes fic, and will always, always love outsider POVs and especially unreliable narrators for any of the pairs here.
General dislikes: explicit sex/violence/gore (anything up to M on AO3 should be fine, but nothing higher, please), consent issues, fics that hinge solely on hurt/comfort or angst/hopelessness.
Greek Mythology
Odysseus/Penelope
Hektor/Andromache
I really love all the fic retellings of Greek myth authors produce, and in past exchanges, the authors I've been lucky enough to have write something for me have usually produced works more amazing than anything I could think to request, so first off, if there is an idea that calls to you, please go for it! If you'd like more direction, though, some notes: obviously, I'm slanting a bit more towards the Trojan War, but--paradoxically--perhaps something that didn't dwell with the accompanying tragedy? Instead, for either--or both--of these couples, peeks at happier times might be interesting.
I love Odysseus and Penelope together, outwitting each other and everyone else around them, so: tell me about their courtship, or their rediscovering each other after Odysseus's return, or even Telegony fix-it fic (because honestly, I can't help but find that a terribly disappointing end for these two). I loved a lot of the Yuletide stuff produced for them on the old archive, from "Spider's Web" and "Incomparably Cunning" to "Turn and Turn Again"!
Hektor/Andromache might be more of a challenge, as their tragedy is basically what makes up most of their story, but I find I'm fascinated by their backstory. I think there's a whole lot of stuff there, from the sack of Thebe to the early years of the war to Hektor and Andromache learning their way as first among the princes and princesses of Troy! Basically, any backstory about Troy before the war would be great!
For both or either couple, political intrigue is something I find fascinating, as well as couples who are extraordinarily competent at ruling/manipulating/getting stuff done, so exploring that would be wonderful, too!
Howl's Moving Castle
Fanny/Mr Sacheverell Smith
Lettie Hatter/Ben Suliman
Howl Pendragon/Sophie Hatter
ALL THE MAD HATTERS! Seriously, though, I find this family fascinating; they start off as a family of four young working class women, and by the end of the book, two of them are witches in their own right, and they've married three of the top wizards in the country, as well as a millionaire. How do they adjust to these changes--CitA lets us know that Lettie and Sophie, at least, seem quite at ease, so how did that happen? So if you'd like to explore that, go for it! Feel free to add Michael/Martha in; I just didn't want to read a fic solely about them, as I feel HMC lets us know plenty about them. But I would also love a solo fic about any of the couples here, and some ideas for those:
First off, however did Fanny and Sacheverell meet and marry so quickly? We know next to nothing about them, and poor Mr Sacheverell Smith barely speaks in canon. Any sort of backstory/behind-the-scenes fic for these two would be really neat!
Lettie and Ben have similar problems; when we leave them in HMC, Lettie has been acquainted with approximately....half?...of Ben and is quite understandably flustered. In CitA, they're married. How does that progression work? Also, Ben is interesting because he has similar origins from Howl, but also seems to deal with them quite differently. How and why?
Which brings us, of course, to Howl and Sophie. ♥ I do appreciate the contrast between their continuous bickering and their grand moments of affections, as well as how well they work together when push comes to shove, but if you'd like a more exact prompt, Howl's POV in CitA has always fascinated me, something that elaborated on that would be really neat!
The Squire's Tales
Lynet/Gaheris
Terence/Eileen
Gawain/Lorie
These books are still one of my favorite versions of Arthuriana, so honestly, anything and everything with their particular style of humor/snark and dorky references would be amazing. I love all six characters listed--except do think Lorie fades rather, about her dramatic first appearance in The Squire's Tale and would love to see her snark with Gawain once again--so I will be thrilled to see fic for this tiny, tiny fandom pretty much no matter what! :D
Dalemark Quartet
Adon/Manaliabrid
Alhammitt/Libby Beer
Robin/Tanamil
As my poor friends probably know all too well by this point, I really love the Undying in the Dalemark Quartet, so any sort of backstory (Who are they? Where do they come from? What truth is there to the stories of how they met--or in Robin and Tanamil's case, what stories have grown around them?) would be absolutely fantastic. But quite honestly, I will probably think anything you write in this fandom, dear author, is brilliant!
The Princess Curse
All
I know I'm signed up for only two of the three pairings, but honestly, this is such a clever, darling book (it might be more geared for the young teen age group, but it's a quick read that is both a fairy tale and Greek mythology retelling, sort of) that I'd actually love to see a more canon-compliant final pairing too! (Er, hopefully that's not against the rules?) What happens next with Reveka and her final choice; AU where things go differently; or more about Vasile and Daciana--anything would be delightful for this fandom!
As a final note, please don't take rambling more about one fandom/prompt than another as some sort of indication that you absolutely need to do one instead of the other; as long as you have fun writing your fic, I am quite sure I will love it! Thank you so much in advance, and I very much hope this helps!