main idea: steampunk AU verse, set in the 1910s-1920s
STORY
The Earl creates machines powered by people's souls, called back by their mourning family, friends, and lovers. The Noah family is a well known old money family, with their hands in everything political.
The time period is the beginning of World War I, and Allen and his twin brother, Robin, are trying to join the British Army, but when their father and younger sister, Julia, dies as in a freak accident on an airship voyage, the strain is too much on Robin. He calls Julia's soul back with the help of the Earl, and so Julia comes back as an Akuma machine wearing Robin's skin. The Akuma attacks Allen when he comes to find his brother, but luckily Cross Marian is nearby and stops the attack--but only after Allen's left eye is stabbed through and his arm ripped off.
Allen manages to convince Cross not to destroy Akuma!Julia/Robin, and instead Cross modifies her to free her from the Earl's control. Cross also creates a mechanical eye for Allen, that can see x-ray style (people's skeletons, not inside clothes, you perverts) and so can tell the difference between humans and Akuma. Allen and Julia are taken in by Cross Marian, who explains the 'secret war' to them.
Freed from the Earl's control, Julia wishes to fight the Earl, and Allen wants to correct the wrong done to his family. The secret army fighting the Earl's machines is called the Black Order, and since the Earl is finding the current political uproar to his benefit, the Order is down on members. Cross fails to convince Julia that she can't fight as a girl, but does convince her to pretend to be "Robin"--this causes Allen distress, and Julia claims it's an insult to their brother, but eventually gives in because it's the only way she'll be allowed. So Julia hides her mechanical nature, and pretends to be the boy who's skin she wears.
After apprenticing under Cross for two years, the pair are allowed to join the Order as Exorcists and given the special weapons designed especially to destroy the Earl's creations and set the soul inside them free. Allen's is fitted to his missing arm, and outfitted with the substance Innocence so it can properly destroy the Akuma.
etc.
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AU INFO
Akuma and Innocence
Major Differences: steampunk'd. the Earl uses a mix of electromagnetism and strange unknown technology to bring the souls back, and trap them inside a specially made case of unknown metal inside the Akuma's skeleton. "Innocence" is an alloy created to pierce that case without exploding it and risking destroying the soul too.
If the Akuma's skin is damaged, it cannot heal, only patched up--sometimes creating a zombie-like look to them. Akuma are also more likely to not just randomly burst out of their skin--since they won't get it back without killing someone and stealing their skin. This is mainly how the Order tracks and finds Akuma.
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CAST
Allen Walker
Major Differences: Allen's eye is not cursed, it's a cool steampunk!eye thing, which he usually wears an eyepiece over to avoid people seeing it's nature.
Relations: twin brother to deceased!Robin, older brother to Akuma!Julia
Weapon: his steampunk arm outfitted with Innocence.
Julia "Robin" Walker
Major Differences: still an Akuma, but the scar in her face is where her Akuma skeleton fit itself into the skin--and how she could take off the skin, the modification is all to her Akuma skeleton--no seal needed.
Relations: older brothers Allen and deceased!Robin
Weapon: ...
Cross Marian
Major Differences: not a magician? he uses awesome steampunk tech instead.
Weapon: a series of badass guns made to fire bullets of Innocence at tank-piercing speeds.
Yuu Kanda
Major Differences: artificial steampunk!cyborg, pretends to be a normal human, but is made to be faster and stronger than a human.
Weapon: a sword made of Innocence to defeat the Akuma.
Lenalee Lee
Major Differences: found as an orphan and trained in the use of the Innocence weapons from a young age. Isn't allowed to leave the Order, because she knows too much now and is too useful. Her older brother joined the Order as a scientist/inventor to keep her sane.
Weapon: she lost the lower portions of her legs in a battle and now has mechanical legs and feet, equipped with Innocence to fight Akuma.
Lavi
Major Differences: still a Bookman, he and Bookman joined the Order for the same reasons.
Weapon: idk
STEAM-POWERED HAMMER?
Krory
Major Differences: After he accidentally destroyed Eliade, Krory is convinced to join the Order (same speech given by Allen).
Weapon: his hands were damaged in the battle and he is outfitted with Innocence claws.
Miranda
Major Differences: After Akuma attacked her town, Miranda shows promise fighting the Akuma when she accidentally destroys a few with an Innocence weapon she picked up when it was discarded by its owner.
Weapon: mainly trained with defensive weapons.
etc.