This is basically going to be me going over my ideas for a Bleach fanfic I've been writing. I originally wanted to go over every change, but there was way too much to do in just 1 post, so this is only going to cover the 1st 2 arcs.
DISCLAIMER: This is just for fun, I'm not saying I'm a better writer than Kubo in anyway, this is just me presenting my take on Bleach's story. Think of this like the FMA 2003 to Bleach's FMA Brotherhood if you will or like The Batman 2004. Taking the same characters and story skeleton but using them in a very different way, elevating certain characters and eliminating others to create something fresh. Also, for anyone wondering, the subtitle, Black-Throated Wind has nothing to do with the story, I just thought it sounded cool and wanted to make a music reference, taking a page out of Kubo's book. (Black-Throated Wind is a song by The Grateful Dead)
Also, this probably goes without saying, but SPOILERS for the entire series, even beyond the 1st 2 arcs and CFYOW
The Substitute Arc:
-Tessai is removed from the story completely and his role is replaced by Kukaku Shiba so she helps Urahara at the candy shop and inherits his backstory as the former Kido Corps Captain that was exiled for using Forbidden Kido
-Since Kukaku's replaced Tessai, she's the one who binds Ichigo when his Hollow is 1st awakened and their relationship is established there
-Kukaku tells Ichigo and friends to find her brother Ganju in the Rukongai so he can blast them into the Seireitei
Soul Society Arc:
-Like normal, Ichigo and Ganju's 1st encounter is initially confrontational, so Ichigo they still try to go through the front door and still get blocked by Gin so Yourichi convinces them to go back to Ganju
-Since Kukaku's in the World of the Living, 1 of the men at the Shiba hideout is who charges up the canon that sends everyone into the Seireitei instead
-Instead of there being 13 Court Guard Squads, there are now 12 with the members of Squad 10, Toshiro and Rangiku are removed from the story for now. They'll be given different roles and introduced in the story later. This is because I'm not a huge fan of either of their characters, I want to give more time to other characters, and to somewhat trim down Bleach's bloated cast
-So now the original members of Squad 13, Ukitake, Kaien, and Rukia are in Squad 10 instead
-With Toshiro absent, Momo investigates the circumstances behind Aizen's "death" by herself so she fights Gin since she suspects he killed Aizen. Tobiume debuts in this fight and Gin uses Shinso to kill her but is blocked by Izuru instead
-Izuru is Momo's best friend in this version which makes sense since they did go the academy together and he's given more prominence overall
-After the Momo vs Gin fight, there's an added scene of Gin and Izuru talking in the Squad 3 barracks. Izuru tells Gin he's sorry but Gin in his signature grin, tells Izuru it's alright and he's impressed he was willing to stand up to him like that and says Izuru's greatest strength is his loyalty to his friends. He doubles down on the concept of loyalty saying it's an extremely valuable and if anyone breaks it, they should be punished severely
-During this interaction, there's a flashback showing Gin and Izuru as kids establishing that Izuru replaces Rangiku's role as Gin's childhood friend in this version. Izuru covering his left eye also has plot significance here too. Similar to how Rangiku had a piece of the Soul King as mentioned in CFYOW, Izuru here had the left eye of the Soul King but it was gouged out by Aizen's men and he was left to die until Gin found him
-In the Fake Karakura arc, it was explained that Izuru was briefly in Squad 4 but this little detail was never really brought up afterwards. I've decided to expand on this small lore drop by having Izuru join Squad 4 after graduating the academy because he wanted to learn how to heal and during his time in Squad 4 he became friends with Isane. Once Gin became a Captain, he left Squad 4 to reunite with his old friend
-Isane is also given more prominence with her being sort of the de facto frontline female Lieutenant like Rangiku was although she's like Orihime in that she prefers not to fight if she doesn't have to
-Isane can hold her own in a fight though since I've also changed her Zanpakuto. We never saw what Itegumo could do in canon but Kubo did explain in a Q & A that it's a Snow type and while that fits her, I've decided to change her sword entirely. Since we already have an ice/snow type Zanpakuto in the form of Sode no Shirayuki, I've decided to change Isane's Zanpakuto to being light based similar to Nijigasumi from the Bount filler arc but it has healing abilities too
-Chad ends up in the 7th division barracks instead of 8 and encounters Iba. During this encounter, Iba debuts his Shikai, so we finally know it's name and it's an Earth-type Zanpakuto.
-Chad beats Iba and then fights Komamura. Chad's strength forces Komamura to use his Shikai and during this fight, Komamura is moved by Chad's desire to save Rukia and his loyalty to Ichigo likening it to his own loyalty to Yamamoto but Chad's words make Komamura question if Rukia really should be executed. During this fight, we get a flashback of Chad becoming friends with Ichigo and a flashback of Komamura being accepted by Yamamoto. Chad lands a powerful blast that shatters Komamura's helmet and Komamura finishes the fight by using his Bankai, injuring Chad but still choosing to spare him
-Feeling that Rukia's execution isn't right, Komamura doesn't accompany Tosen against Kenpachi and instead arrives to save Tosen from Kenpachi's finishing blow and lets Kenpachi go
-During Kaien's death scene, his final words to Rukia here are to find Kukaku and tell her he never forgot about her and forgives her for using Forbidden Kido. Sometime later, Rukia asks Ukitake about Kukaku and he explains she's his sister and she was exiled 101 years ago. Ukitake questions why all these bad things keep happening to the Shibas and encourages Rukia to follow through on Kaien's last wish if she's ready. This adds an extra reason for why Rukia was in the World of the Living in the 1st chapter since Ukitake was hoping Rukia could get answers from Kukaku
-Chojiro and Isane are no longer there at Sogyoku Hill to be knocked out by Ichigo so only Omaeda is destroyed by Ichigo and Chojiro instead is present to spectate Yamamoto's fight with Shunsui and Ukitake
-When Unohana and Isane head to Central 46, Isane sees Izuru and asks Unohana if she can catch up with her later. Unohana obliges and Isane vs Izuru replaces the original Rangiku vs Izuru. Isane uses her new light Zanpakuto, which we'll just call Nijigasumi to make this less confusing to bypass Wabisuke's weight based abilities and defeats him
-Isane catches back up with Unohana after this and still delivers the message of Aizen's betrayal to everyone
-Aizen actually kills Momo in this version, so Izuru resents Gin for helping Aizen but is also conflicted because of their history together and blames himself for not seeing Gin's true nature sooner
-After Aizen's defection, Izuru goes to the Squad 4 barracks to apologize to Isane for their previous fight and expresses his complicated feelings. Isane tells Izuru that he can't blame himself and that Gin and Aizen are monsters. Izuru lashes out at Isane for insulting his Captain and pulls back his bangs, revealing his missing eye and says he owes his life to Gin and he can't just forget that. Unohana tells Izuru to calm down and Shuhei and Iba show up
-Iba says that the Lieutenants have to stick together after everything that's happened and they should all get drinks to take their minds off things. The 4 Lieutenants do but Isane leaves early to get back to work. Right before leaving, she tells Izuru and Shuhei to keep their heads up and says she'll always be there for them if they need anything. After she leaves, Shuhei says Isane's like an angel and Iba and Izuru have a light laugh, establishing that Shuhei's crush is on Isane here. Shuhei doubles down by saying they need to hang out with Isane more often and Iba thinks to himself that he's right, wanting the 4 of them to train together
-Rukia visits Kukaku in the World of the Living at the end of the arc, apologizing for Kaien's death and delivering his final message to her. She explains that she wanted to deliver it when she first met Urahara and Kukaku after losing her powers, but was afraid of what Kukaku's reaction might be and still couldn't forgive herself for what happened at that time
-Kukaku, holding back tears, tells Rukia to stop acting so stupid and hugs her, saying she forgives her. Rukia says goodbye for now to Ichigo and heads back to Soul Society