I cast unholy awakening! Arise from the horrors of the forgotten! Let the rage flow in unstoppable might!
Yeah, a idea have been bothering me for a while and this thread was the closest one I could find.
The plot starts as canon. Fishbone D attacks the Kurosaki clinic, Rukia tries to stop it but fails, and as a last resort gives her powers to Ichigo, who gains Shinigami powers.
The catch? He don't get Zangetsu.
He gets a original zanpakutou, one that surprise him:
A pocket chainsaw (note, I am not talking about toys here, the pic is just to help with mental images)
Ichigo is shocked at getting a tiny little chainsaw and the hollow mocks him, they fight and Ichigo manages to win. He is disappointed but begins to work as a Substitute Shinigami, fighting weekly hollows, befriending the gang, all the canon stuff.
Then shit gets real due to Byakuya & Renji have arrived, arresting Rukia for treason, and Ichigo, trying to stop them is beaten brutally by Byakuya and gets saved in the last second by Urahara, who manages to barely make Byakuya & Renji retreat with Rukia back to Soul Society.
Urahara tells Ichigo that certain death awaits Rukia due to highly unstable political situation in Soul Society, which Ichigo completely ignores and asks desperately the man with the nice hat for power to save his friend Rukia.
Urahara reluctantly agrees to train Ichigo and after some quick haxxoring & cheating he teaches Ichigo Shikai.
Which looks like something similar to this:
Which has the ability like Gin's zanpakutou to lenghten and shorten but not as fast or as long as his can do.
So armed with his new shikai he and his merry gang of friends invade Soul Society. Which is in political chaos, due to threat of Baraggan & his army of hollows in Hueco Mundo + shitloads of treasons from various Shinigamis that all have their own reasons for that. Both Gin and Kenpachi I picture belong in this criminal group for their liking of killing others.
So Ichigo and his merry friends run around in Soul Society, trying to find Rukia while beating various mooks. When they take a rest in a bar, they meet Gin (who they have no idea who he is), and since he is interested in people with similar zanpakutou like his own (let's say that people can sense zanpakutous that are similar to their own) and after some idle chitchat gives Ichigo & comapny an offer:
He offers the knowledge to achieve bankai in exchange for them seeking him for a duel later. (lets say that everyone with spirit powers can achieve bankai, but most lack the capacity to control it, hence few learn it due to the risks) but since Ichigo have no idea about that he accept.
Cue short training arc where he somehow manages to gain and control bankai.
Which looks similar to this:
Which can saw through everything with Reishi which it absorbs which it uses to generate an aura of terror and the more reishi it absorbes to more powerful the aura becomes. For example, Ichigo activates the bankai: opponent feel a chill in his back. Add a few minutes of reishi absorbation and opponent is paralyzed in terror.
They find Rukia, who is guarded by Byakuya. Ichigo manages to free her, and then hold off Byakuya with his shikai. The rest run. Byakuya activates bankai to pwn Ichigo. Ichigo copies with his bankai. Cue epic fight. Ichigo wins thanks to bankai aura. Ichigo & gang run away.
Add then some arc where they go to Hueco Mundo & shit.
There, a short story.
TL;DR: Original zanpakutou. First form is pocket chain saw, for easy storage and swift and light movement.
Shikai is chainsword. Is able to lengthen and shorten its length like Gin's zanpakutou. To lazy to come up with an activation command.
Bankai is a chainsaw. Saws through everything made of Reishi and absorbs reishi to project and aura of terror onto opponents. Ichigo can use this form for like 10 minutes or something.
Sorry for the necro.