I talk too much! Mostly out of sheer enthusiasm, but please don't be intimidated. I've thrown out a bunch of prompts for each character in each fandom but if none of them speak to you, provided you're not specifically writing something I mentioned as a squick, write whatever you want. These are mostly small fandoms that don't see tons of fic, so I'm thrilled that you're considering writing a little trick or treat for any one of them.
Diabolik Lovers- Komori Yui and/or Sakamaki Kanato
This is my guiltiest of guilty pleasures and if we have matched on it I SALUTE YOU for joining me in this hand basket. My number one stipulation is to please not bash Yui or imply she's stupid for not escaping or whatever. I love Yui and I think she acquits herself pretty admirably for the most part and basically gets Stockholm'd out of pure survival instinct. This goes for both anime and game canon, though the Stockholm is a little less obvious in the anime. Also if you write Kanato/Yui, please don't make it totally uncreepy and legitimately sweet.
I am also totally okay with Kanato on his own if that is what you'd rather write! Or Kanato messing with Yui in a not necessarily shippy way. Or Yui attempting to get Kanato to like her in order to improve her situation.
If you are down for it, I'd love to see Kanato and Yui as a couple who cannot possibly do normal couple things because they're both so thoroughly fucked in the head. Dub con is fine-- heck, non con is fine (though considering some of Kanato's comments maybe more of a mind controlled and unable to move kind of deal rather than a physical struggle) knife kink is great, bloodplay is great, praise kink and body worship are great (Yui to Kanato more than the other way around) and yeah, it's a yandere vampire and his prey. I'd be totally fine with completely vanilla sex with emotionally unhealthy creepiness. Or emotionally unhealthy creepiness with no sex, or with just the mention/implication of sex. Sex is very much not a requirement!
If nothing I've mentioned so far appeals, I'm happy to see a snapshot of backstory! Besides the unpleasant stuff we already know about, what was Kanato's childhood like? What are his specific beefs with his siblings, and when was the first time he thought about killing each one? Was there a time when he really loved the twins, or felt close to Reiji and Shu, or did he ever look after Subaru? Were there other girls before Yui? Are those dolls actually corpses he used to know? Are they actually dolls and he just has a creepy fucking imagination? When did he 'meet' Teddy? Alternatively, what was Yui's childhood like? Did she ever have any moments where Cordelia tried to manifest when she was younger? Was she ever lonely, or was she actually quite happy with her life up until she was dumped on the Sakimakis' doorstep?
Fire Emblem: Rekka no Ken- Erk and/or Serra and/or Fiora and/or Eliwood
I really love these four so even if you've just matched on one of them we're good to go. I'm happy with something for any of these individuals, or any two or if you want to write three of them but not the fourth that's also fine.
Erk- for gen, something about his past or learning with Pent or how being with all these people changes him, or him becoming friendly with any of the other four. For ships, I think Erk/Serra is hilarious, and Erk having a crush on Eliwood could totally be a thing. Erk/Pent/Louise would also be excellent (kind of incest-y but that's okay) as would Erk/Canas, and I've warmed up to Erk/Priscilla.
Serra- for gen, something about how she spent the year's between Lyn's story and Eliwood's story would be neat, or a friendship story with Erk, Eliwood or Fiora (or if you're willing to write non nominated characters, Lucius, Hector, Matthew, Florina or Lyn). For ships, Erk/Serra is great, Eliwood/Serra could be fascinating, and I'm also very down for Hector/Serra, or Priscilla/Serra.
Fiora- for gen, something with her sisters and the pegasus knights, maybe? Or a friendship story with Eliwood, Serra, Erk or if you're willing to write non nominated characters, Isadora, Louise, Pent, Rebecca, Renault or Harken. For ships, I adore Eliwood/Fiora and see potential for Vaida/Fiora or Isadora/Fiora.
Eliwood- for gen, maybe something post-game dealing with becoming Marquess and/or becoming a father, or friendship with Fiora, Serra or Erk, or the tactician (male or female). For ships, I love Eliwood/Fiora but could also be down for Eliwood/Harken or Eliwood/Serra or Eliwood/Erk or, heck, Eliwood/Pent/Louise.
Fire Emblem: The Sacred Stones- Lyon and/or Knoll and/or Eirika and/or Ephraim
I really love these four so even if you've just matched on one of them we're good to go. I'm happy with something for any of these individuals, or any two or if you want to write three of them but not the fourth that's also fine.
Lyon- definitely leaning towards trick for this guy. What was the nature of the research he did with Knoll? Where does Lyon end and where does the Demon King begin? What would have happened if he was stopped earlier? Does he secretly write erotic friend fiction about Ephraim and Eirika?
Knoll- I love how matter of fact he is about dark magic and how using it doesn't mean you're an immoral person. I particularly like his conversations with Lute and Natasha but since neither were nominated characters obviously you don't have to write them. I kind of wonder why he didn't speak up against Lyon earlier, and if you're up for it, I could totally see Lyon/Knoll having been a thing.
Eirika- is one of my absolute all time favorite Lords, and I love her, and I want her to be happy.. Any gen about her- post game, childhood, questioning her decisions during the game- is more than fine. For ships I love her with Forde, Innes or Saleh, and I am also down for some fucked up codependent twincest. If you want to go down that route I'd prefer no Ephraim being condescending/patronizing to Eirika and making decisions for her, and I'd also prefer it not to be a totally sweet fluffy thing that they're open about and no one thinks is weird or creepy. I am down for it being a one-sided thing (just Eirika crushing guiltily on Ephraim or just Ephraim crushing guiltily on Eirika) or a mutual secret.
Ephraim- is one of my other all time favorite Lords! For gen I'm interested in how he adjusts to ruling Renais since it kind of seemed like at the beginning of the game he was running away from the responsibility. Or heck, maybe he decides it isn't for him and passes it on to Eirika. Or maybe he and Eirika end up sharing the throne. Or something from his childhood or learning how to fight or reflections during the game. I have thoughts about twincest up in Eirika's section if you want to do that but if you don't, I am very happy to read about his marriage to L'Arachel (how did it "throw Rausten into turmoil"?) or a relationship with Kyle, if you're up for writing him though he wasn't nominated.
Fire Emblem: Awakening- Tharja and/or Male or Female Robin
Tharja- Spooky dark mage stuff! I'm happy for something with her magic, or something with her cursing somebody she doesn't like or planning to curse somebody but deciding not to. I'd also be interested in family fic with her and Noire-- does she become a better mother in the better future? For Noire's other parent, I like Stahl, Gaius, Libra or Gregor best. Or you could write something about her unrequited obsession with Avatar/Robin of either gender (I mostly prefer it unrequited but I could be down for it being briefly requited providing this didn't make Robin Noire's parent since I don't buy Robin could ever be the totally cowed parent Noire describes)
Male Avatar/Female Avatar Robin- I'm combining these two because I really don't care whether you make Robin male or female, choose whichever works better for your piece. I'm curious what Robin's life was like before the amnesia-- did they do bad deeds or were they raised as a basically good person far away from Validar? Or did they end up working for Validar for a little while before getting fed up and leaving the country and then lose their memory? Or some other thing! Or you could write something post game, or something in an alternate version of events. Or you could have one male version of Robin meet a different female version of Robin! If you want to write something shippy, at the moment I like Robin with Cherche, Sumia, Tiki, Henry, Priam or Ricken. Or, what the hell, Robin on Robin action if you want to!