This would actually be a really cool rewrite and a really cool way to fully flesh out the story while also giving it an interesting end. I like the change in Yhwach's motivation, the deal he made with Yamamoto. It makes it more compelling and also kinda makes Ichigo's powers less Gary Sue-esque, I guess. Him having a part of Hogyoku, a part of the Soul King within him, thus stabalizing his soul when he was a child because I'm pretty they said somewhere that he should've been torn in the womb for being a hybrid mess of Shinigami-Hollow-Quincy-Human. His powers kind of being sealed so he could a human was Kisuke's wish for Hogyoku so it did that but over time, Ichigo grew stronger and more aware of spirits.

Rukia and the Hogyoku Aizen planted inside her kickstarting his powers makes their whole friendship way more awesome as well because they're actually connected in a spiritual way. It makes Aizen and Ichigo's final fight more impactful-- the two halves of the Hogyoku fighting

Ichigo sealing his own powers away after Aizen is gones but slowly getting them back. Ginjo hearing about him and his powers and becoming interested. He wants to steal the Hogyoku in Ichigo but ends up only stealing a portion of it. Ichigo, believing that he had now lost his powers indirectly sealed them once more and Rukia comes back to kickstart it once more because even if the half of the hogyoku that had been in her is gone, some of it's reiatsu lingered and that came through in their connection along with all the other captains reiatsu

Ichigo gets his powers back and blah blah blah but rather than Ginjo training Ichigo and all that shit, it's more of Orihime and Chad's arc where he only joined because he was unwilling to leave them to this weird shady guy that wouldn't leave him the fuck alone

The whole thing about the zero squad and Yhwach using Ichigo to find where them and the Soul King were hidden is a really cool idea as well

Having the Soul King actually have a consciousness rather than being sealed away in an orb and a mutilated body kinda sounds like a cool idea too. Him making the conditions of the SS better (especially for RUKONGAI, yes I'm fucking bitter about that)

I'd be content with that sort of ending but if you wanna include the hell arc you can still do all the same shit as canon cause I'm pretty sure it was stated in one of the light novels that the Soul King didn't do anything to Hell beyond closing the gates

But in comes Kazui the even weirder hybrid. Who apparently sends innocent souls to hell behind his parents back??

But whatever. Yk those creepy hollows from hell that we saw in the No Breathes From Hell one shot? Yeah, I want Orihime's mom to have been attacked by one of those guys and that's how she became a fullbring. Yes, I'm doing that because my girl deserves actual screen time and some explaination on how exactly she became a fullbring just from proximity to Ichigo's reiatsu. If that was true, Tatsuki, Mizuiro, and Keigo should've gotten powers as well.

Anyways so some weird soul/genetic bullshit happened and Kazui, rather than having similar hollow powers to his dad, is a Shinigami-Hell Hollow-Quincy-Human (Fullbring I guess??) hybrid.

That's why he can open the gates of hell the way he did.

Also him taking a part of the Hogyoku from Ichigo in order for his soul to be stabalized

(Wow, the Kurosaki line is so fucked up when you think about it. If Kazui and Ichika are a thing like Kubo was hinting at, the soul situation of their offspring would be so fucking weird)

Anyways back to what I was saying, whoever the antogonist of the Hell arc is probably manipulating the kid and teaching him some fucked up shit and that's why his parents seem to have no clue

I'm pretty sure Hell is said to be an actual entity so maybe it's using Kazui in order to upset the balance because he's sending souls to hell right? Idk why it would do that but it'll be like an army and all the dead captains, arrancar and quincy.... yikes, they'll be facing the original gotei and all the other dead captains and Kenpachis

Since Yhwach is kinda gone in this rewrite you wouldn't have Kazui absorbing/destroying/what ever he did to Yhwach's soul wouldn't happen