Another Fire Emblem mixer of mine, and it's another Three Houses/Hopes fic that sees the addition of another house into Garreg Mach's environs, though in this case, the students making up the house...
...are based on Batman and the Outsiders. Like the Ashen Wolves, they're a mix of faces from all over the continent. There is a decent amount of blending of the Bat-mythos (and a few other DC elements) into the story of Houses/Hopes, and the presence of a Batman-like figure does indeed upend a number of plots (and even prevents the death of Jeralt). The new house in question, called the Gray Ghost house (a nod to Simon Trent/the Gray Ghost from Batman: The Animated Series), consists of:
-House leader Bryce of the Wainwright dukedom in Faerghus; like Dmitri, his parents were casualties of the Tragedy of Duscur, only they were deliberately targeted by one of the instigators for their inadvertent discovery of the plot in question. Seeking to find his parents' killer and the one who hired them, in the years leading up to his time at the Officers Academy, Bryce trained himself to peak physical perfection in body and mind, and adopted a masked persona known as the 'Bat-Man' to terrorize the bandits and criminals running throughout the land, as well as dismantling the operations of a mysterious group of dark-robed mages. Based on Bruce Wayne/Batman. Class progression: noble--myrmidon--lord--hero
(The first appearance of the Bat-Man in a hypothetical story would be during the first encounter between Jeralt and Byleth and the house leaders, with one of them noting Bryce's absence. It's then we see that Kostas has, unfortunately, run into the Bat-Man; their fight and dialogue pays homage to the initial appearance of the Caped Crusader in Tim Burton's 1989 Batman movie.)
-Alfredo, heir of a dissolved minor duchy within the Adrestrian Empire, and longtime friend and aide to Bryce, acting in such a role akin to Hubert or Dedue. Serves as voice of reason among the Gray Ghosts, and is something of a neat-freak. Has a bit of a history with Constance of the Ashen Wolves, and is good friends with Ashe and Marianne. Based on Alfred Pennyworth. Class progression: noble (minor house)--monk--priest--bishop
-Brion, heir to the Markova territory within Faerghus. Attends the Academy in the hopes of bettering his political and tactical skills so he can better protect his brother, Gregorio, from the ambitions of their treacherous uncle, an Empire-aligned noble by the name of Fyodor (based on Baron Bedlam). Also seeks to find his missing younger sister, Tara (who, like Edelgard, Emile, and Lysithea, was abducted by 'Those Who Slither in the Dark' and experimented on). Based on Brion Markov/Geo-Force. Class progression: noble (minor house)--fighter--armored knight--fortress knight
-Geoff, son of a Leicester merchant family, with aspirations of becoming a great teacher. Possesses a strong sense of truth and justice. Based on Jefferson Pierce/Black Lightning. Class progression: commoner--soldier--cavalier--paladin--dark knight
-Tatsumi, daughter of the head of a minor duchy in Dagda whose family relocated near Fraldarius territory in Faerghus, and a sword-fighting prodigy who trained with Felix; dreams of besting Catherine in combat. Second cousin to Shamir. Based on Tatsu Yamashiro/Katana. Class progression: noble (minor house)--myrmidon--mercenary--swordmaster--mortal savant
-Gabrielle, adopted by a prominent teacher at he Fhirdiad Royal School of Sorcery, and as such studied in the same class as Annette and Mercedes. Has very little memory of her life before her adoption. Based on 'Gabrielle Doe'/Halo. Class progression: commoner--monk--priest--bishop
-Lia, daughter of a minor Leicester noble family who combines both beauty and brains, and tries especially hard to downplay the former. Based on Emily Briggs/Looker. Class progression: noble (minor house)--monk--mage--warlock--gremory
-Jean-Paul, scion of a minor Adrestian noble house who, while appearing bookish for someone of his size, turns into an absolute beast on the battlefield. Lia is the only one who can manage to calm him down. Following the time-skip, Bryce gives Jean-Paul his own suit of armor that allows him to go toe-to-toe with the Death Knight; in this guise, Jean-Paul is known by the moniker of the 'Avenging Angel'. Based on Jean-Paul Valley/Azrael. Class progression: noble (minor house)--fighter--brigand--warrior--war master
Additionally, the Gray Ghosts have their own answer to Cyril in Ricardo (based on Dick Grayson/Robin), an orphan who Bryce took in with the help of his cousin, noblewoman Katrina (based on Kate Kane/Batwoman), and works as a page at Garreg Mach while Bryce and Alfredo are there. Following the timeskip, Ricardo becomes the squire to the 'Bat-Man' under the Robin guise. All the students in the Gray Ghosts are aware of Bryce's nocturnal activities, and as such cover for him; he has promised them, in turn, with assistance in gaining justice for their own respective mysteries or tragedies.
Some familiar characters in Three Houses/Hopes serve some additional roles in this take as well:
-Gilbert/Gustave is their analogue to Commissioner Jim Gordon, with Annette, accidentally stumbling onto Bryce and co.'s secrets, becoming their equivalent of Barbara Gordon (and, following the time-skip, secretly a Valkyrie version of Oracle).
-Dmitri, given the dual sides we see to him in Three Houses, draws some parallels to Harvey Dent/Two-Face.
-Claude, in this take, would be something of a trickster pastiche of Green Arrow, though without donning a costume himself.
-Emile/Jeritza/the Death Knight is analogous to the Reaper, the villain of 1987's Batman: Year Two, as both are garbed in skull-faced outfits and wield scythe-based weaponry while also being affected by madness, as well as having family who take up a religious calling (Mercedes/Rachel Caspian).
-Those Who Slither in the Dark/the Agarthans fills the niche of the Court of Owls.
-Edelgard, in a sense, is the story's equivalent of Lex Luthor, given their slightly-similar personalities and drive to humble the powerful, even by acquiring more power themselves. As the Flame Emperor, she squares off with the Bat-Man on a small number of occasions.
The story comes to a head when Rhea takes Byleth and their class (in this case, the Blue Lions) to the Holy Tomb to receive a revelation; when the Flame Emperor confronts them with Metodey and their soldiers, before a single weapon is raised, Bryce/Bat-Man knocks Metodey's sword out of his hands with a batarang and leaps down in-between both sides, where he then methodically and immediately exposes Edelgard as the Flame Emperor. Bolstered by the hidden Gray Ghost students, Byleth, Rhea, and co. actually manage to apprehend Edelgard before Hubert can warp her away.
Bryce/Bat-Man convinces Edelgard to come clean about the reasoning behind her actions, having brought in Hubert to corroborate her story. Edelgard, realizing she'd gone about everything the wrong way after an intense conversation with Bryce, spills the beans about the Agarthans, their plans for Fodlan, and their masterminding of the Tragedy of Duscur. Unfortunately, 'Slither' has managed to completely take control of the Empire, causing Edelgard, Hubert, and the other Black Eagles to seek asylum in Faerghus. Following the time-skip, it's now all five houses and the Church vs. the Agarthan-controlled Adrestrian forces for the fate of Fodlan.
Following Byleth's return after the time-skip, she finds that Bryce and the Gray Ghost students have formed an anti-Agarthan resistance force, which has grown to include other former Garreg Mach students and a surprise addition. Most surprisingly, some of the crest-less students have unexpectedly manifested abilities (in a sense, their 'meta-genes' have activated). Some have taken up familiar DC mantles:
-Petra is the story's version of the Huntress.
-Dorothea, after manifesting a powerful sonic scream, becomes the story's Black Canary.
-Balthus dusts off the masked alias used by one of his former mentors in the fighting arts, becoming the story's Wildcat.
-Ignatz, after an unexpected encounter with dying alien Abin Sur, ends up becoming this world's Green Lantern. (He, in a sense, is a parallel to another notable artist with a Power Ring in Kyle Rayner.)
-Ingrid, after meeting the mysterious wandering knight known as Sir Justin, becomes Fodlan's own Shining Knight.
-Leonie, over the five years Byleth was gone, found her hidden heritage kicking in as she grew almost as tall as Raphael and Dedue and put a bit more muscle on--due to finding out that said heritage was her being descended from a tribe of woman warriors known as the Amazons. (Judith, Catherine, and Shamir are also revealed to have some Amazon blood in them as well.) She fills the role usually held by Grace Choi.
-Astonishingly, Edelgard has joined their number, as the story's version of Red Hood.
Additionally, a big change during the timeskip is that, thanks to the pooling of scientific know-how between Lia, Geoff, Hanneman, Linhardt, Constance, and Lysithea, they were able to extract the extra crests from both Lysithea herself and Edelgard, giving them back what years they lost in the Agarthan crest experiments. This pool of knowledge was also able to separate Arval/Epimenides from Shez (female), but in the process she retained the superhuman speed she'd been endowed with, making her the story's version of the Flash.
In both the Academy and War phases of the story, additional villains do appear based on some prominent Bat-rogues. The Agarthans have a strategist who's essentially the Riddler as a Trickster, a specialist in ice spells based on Mr. Freeze, and a magic user with expertise in fire spells based on Firefly. An Agarthan experimental subject is essentially a brawler version of Bane. Two demonic beasts that were also experimental subjects that they unleash are based on Killer Croc and Man-Bat. Outside the Agarthans, in addition to Pallardo, there is a Fodlan version of Oswald Cobblepot/the Penguin and a ruthless mercenary troop leader based on Black Mask/Roman Sionis. Ra's Al Ghul and the League of Assassins are here portrayed as being from Almyra. And of course, the most notorious of the foes faced by the Gray Ghosts and their fellow Garreg Mach alum is an insane prisoner the Agarthans only bring out for the truly dirty jobs: an individual in the assassin class known simply as 'the Laughing Man'. (Kronya, who survives in this take, is in a sense his Harley Quinn.)