Hello, Yuletide writer, and first of all thank you, thank you, thank you for even attempting to write one of my fandoms! I tried to write the same amount about each of them, because I truly would be happy with fic for any of them, but I'm afraid I didn't quite succeed, so you'll have to trust me when I say I really do love them equally and am super pumped!
On DNWs: I decided it was most helpful to provide DNWs on a request-by-request basis, since sometimes a 'yes please' in one fandom is a 'please no' in another and all that. In some cases I've listed elements that are present in canon as a DNW. Generally what this means is I don't want it to play a major part, be detailed, or the focus of the story, not that you need to pretend it doesn't exist/never happened (unless for your own reasons you'd like to).
Blanket DNWs that are a bit more relevant to some fandoms than others: terminal illness, non-canonical major character death, pregnancy, kidfic, total paralysis/I have no mouth but I must scream kind of scenarios, amputation/permanent disfigurement, ageplay/extreme underage, vomit, scat, beastiality, insane asylums, shock treatment, medical stuff.
Also my AO3 name is HannaM just in case you're a lovely person browsing for treats.
Diabolik Lovers - Komori Yui, Sakamaki Kanato
I've requested Kanato and Yui both because they are my favorites and because I ship them like burning, but if you don't ship them I'm still super down for more anime compliant predator/prey stuff and pre-relationship dynamics. I loved that the anime kept one of my favorite Kanato moments, which is when Yui's protesting that being bitten hurts and she doesn't like it, and Kanato actually registers this, then decides to make it feel good for her instead. I really like that even in a universe where she's not his sole possession, that's still something he feels the need to do.
I also like the implications that Yui has some things in common with Kanato (she wants to buy clothes for Teddy too!) especially when they're sort of dark things. Sure, a lot of her weirder sentiments come late in the game when she's been around Kanato a lot, but Yui's not exactly a sponge. I find it hard to believe that the dark stuff wasn't always in her to a degree, just carefully repressed. One thing I find tragic about Kanato's route is that I absolutely believe Yui does fall in love with him, but he can't believe it himself, and his refusal to accept her love on her terms (the more he loves her, the more frightened he is that she'll leave him, and the more he feels the need to control her so that won't happen) ultimately makes things worse for both of them.
Going back to the anime, I'm interested in just what the Sakamakis think they're doing. Is it accepted that Yui is the bride of all of them, or is there a bit of a competition going on for who can really sink their hooks into her? Or can they just not agree and everyone has a different idea about whether this situation can possibly last? Does Yui ever compare her experiences with the brothers in her mind/feel any particular leaning towards some of them being less difficult than others or are they firmly all the same to her? Is there any chance of her managing to change her status or wrest greater control over them?
Should you be interested in writing sex, these two characters have a lot of it and I would certainly not be opposed to them having more of it! Creepy dubcon vanilla sex is great (that basically describes their first time anyway) kinky sex is also great, and basically I'll go into more detail in my Yuleporn comment if this is relevant to your interests.
Diabolik Lovers DNWs: total paralysis, pregnancy or kid!fic, outright graphic non-con, Kanato or Yui dying permanently, totally uncreepy vampire romance, Yui's stuff/room being destroyed to punish her, specifically public humiliation on the street/in front of classmates or humans and I'd rather not have heights brought into it unless everyone involved can fly.
Heathers (1989) - Veronica Sawyer
I was super excited to see this movie in the tagset, oh my god. I watched this movie a lot as a melodramatic freshman in high school back in the day okay. That said I haven't read much fic or thought about it in a while so I'm pretty open to a wide variety of directions here! What did Veronica have to do to get in with the Heathers initially? Or for that matter, what did she do to become 'friends with' Heather Chandler, since presumably McNamara and Duke would have followed her lead? There must have been some perks beyond just literally being popular to joining their group, so what were Veronica's reasons? Are there things she's done (besides the obvious whole murder thing) to other students that she regrets? Will the Sawyers ever notice something's been going on at the high school?
To go in a more shippy direction, I do enjoy Veronica/JD though I'm also perfectly happy with how their relationship er, terminated in canon. I'd be interested to see some more of the good times, and the 'starting to notice a lot of creepy things and feel weird about this relationship but not ready to call it quits just yet' times. Presumably they had a few more dates we didn't see, and JD managed to do some serious convincing plus playing up her anger and frustration at others. So maybe more JD as Lady Macbeth moments, you know? I'd also be interested if you wanted to take Veronica and Heather Chandler to a messed up femslash place, or have Veronica discover Heather's coercing one or both of the other Heathers into sex in exchange for other things.
Heathers DNWs: JD forcing himself on Veronica (or JD forcing himself on anyone), any of the characters having a baby (abortion and/or referenced abortion is fine), focus on Heather McNamara's eating disorder/any other characters suffering from disordered eating, Veronica being publically humiliated (privately humiliated/in front of the other Heathers is fine).
女王蜂の王房 | Jooubachi no Oubou - Kaguya, Rin
So I somehow got sincerely emotionally invested in humanoid bee woobies who live in a world that is dystopia levels of fucked up, with a matriarchical society that practices polyandry (read: each princess has a drone harem) as an excuse for constantly kinky sex. Humanoid bee woobies that are fundamentally kind and generous but filled with self-loathing and too screwed up by their past and present fucked up situation to not do terrible things to the ones they love! Really, truly, terrible things that are often also hot uh I mean what.
So one of the things I find really interesting about Kaguya and Rin's dynamic and would always be delighted to see more of is the degree to which they see themselves in each other, and where that empathy is helpful versus where it interferes with their understanding of the other as a separate individual person. Kaguya clearly sees her own precious childhood in Rin's "innocence" and is very protective of him because she couldn't protect herself, but Rin seems to identify more strongly with Kaguya's fierce desire for survival and revenge, and the more she tries to wrap him up in blankets and take care of him the more frustrated he feels and the more desperate his desire gets to prove he's not a pet. I'd also be specifically interested in more of the aftermath of the scene where Sumeragi goads Rin into sexually assaulting Kaguya with him. There's a whole lot of horrifying and sad implications in Rin's confusion over the idea that Kaguya's body responding didn't mean she wanted that, especially given the way he flees when Kaguya bitterly decides to force him too. And then there's the whole setup with the magical eye to play with, because I'm hardly going to rule out porn and violent stuff from a canon where that's so integral. Just... with the below reservations.
Jooubachi no Oubou DNWs: Piercing/needles focus, vore, violent stuff involving eyes and fingers, pregnancy, extreme underage/ageplay, heavy focus on Sumeragi, total paralysis.
Nameless ~the one thing you must recall - ???, Eri
Firstly, I realize it's kind of difficult writing for a character with no canonical name, but I can't say I have any specific preferences so if you very understandably need to name the guy in your story, feel free to choose something thematically appropriate or whatever and I'll cheerfully go with it!
I felt a bit cheated of a full crazypants route with ??? because I am ridiculous, so anything further exploring ??? in full human form attempting to be Totally Definitely A Fellow Human Student would be delightful, with or without continued assurance that he's Eri's boyfriend. Or further exploration of one of those many bad ends (except maybe not the one where Eri is a doll because full paralysis/trapped inside a body silently screaming is something of a gut level horror for me) with ??? looking after Eri with creepy tenderness/tender creepiness? . Or further exploration of ???'s end of things while Eri was on other character's routes! How does he feel about Yeonho and Tei trying to break her before he gets a chance? There's also a lot of different (creepy!) places you could go with the whole memory thing, especially since on some of the later routes even the player doesn't remember certain days.
Or to get a little less dark, post second secret end with ??? trying to fit in at Eri's place with the other former dolls? They'll likely have some hostility towards him considering all he's responsible for, so it probably won't be easy. Or, what if Eri is seeing one of the others romantically (my preferences being Tei and/or Yeonho) and ??? has to deal with feeling like he's being left behind again even though she still cares for him? Or the flip side, how does Eri feel about having to juggle all of the dolls plus ??? having definite feelings for her and caring about all of them but not knowing where to go from here?
Nameless DNWs: Outright graphic non-con, permadeath of Yeonho or Tei, Eri being bullied by the fangirls, Eri as abuser/rapist, fully paralyzed Eri.