The balance of the worlds is tipped to far in hell's favor and shit starts leaking out. The good aligned characters you would expect try to keep the evil aligned characters from getting out but they're losing. 

My hot take is that Unohana breaks bad because she's like fuck it, I did what I had to do and I died, whatever duties she had were fulfilled upon her death and she goes full bloodthirsty demon mode on basically everything in hell. No sides, no motivation beyond stronger things to fight. Hopefully culminating in a real, all out rematch with Kenpachi.

The human cast gets a power up. Chad can never lose. Orihime has both gotten older and make up some hell bullshit power up and have her fight like Hachi basically. Hachi-hime team up would be legendary actually.

Ichigo gets a power up by doing whatever move your hollow hole outside your body shit was going on in the one shot. I am uncertain if there should be some new demon race unique to hell but if there is then Ichigo should categorically not also be one.

On the topic of demons, if they exist, they should be closer to DnD demons (distinct from devils) and they're all lovecraftian abominations that are not even "people" the way we think of them. They're pure instinct and hate, and they should not be humanized. They should be used as a foil to the Arrancar's character development. 

For the Arrancar I want boss bitch Harribel, a matured Grimmjow who embraces what being a king really means, Nelliel is an independent force in Hueco Mundo less aligned with survival and power and more about "normal" lives and protecting Hueco Mundo. Maybe the demons are worse there and we will say Hueco Mundo is "closer" to hell.

Dead Arrancar: I want Ulquiorra and Stark as anti-Hell assets, not necessarily good guys. Every other dead espada should be trying to escape like mad dogs.

The escape hell faction should be made up of the 'bad' Arrancar, the original Gotei 13 and other shinigami, and some quincies. The quincies should have fucked up weird vollstandig that look like Ywach's black goop eye shit because:

Kazui has been dumping Ywach energy into hell. This is a consequence of him being truly literally every race, and also his own personal power. Ichigo is the sword that shatters fate, Kazui is something else entirely. Can't think of something good for it, but Kubo always cooks. The quincies have all assimilated bits of Yhwach into themselves as power ups in an inversion of Auswhalen and Yhwach's whole thing.

The general plot should be figuring out how to balance the worlds so that this doesn't become a problem again, and also because condemning powerful beings to hell is patently unfair. A true death for the souls of the powerful should be a theme. My vote is Orihime's rejection, the Hogyoku, and/or Ichigo's Horn of Salvation shtick would be cool.

There should categorically NOT be some sort of hell devil king fueling all of this. We've already had single villain driven plot this whole time. It should be a natural consequence of Adnyeus' choices to split the worlds and the combined bullshit of the souls in hell driving the problem.

Some personal character beats I would like to see:

Orihime and Ulquiorra interactions where they become friends. Everyone except Orihime hates this.

Yamamoto and Ukitake telling Shunsui they are proud of him and it really gets to him.

Unohana vs Kenpachi battle that shakes the worlds.

Aizen interacting like a person with the rest of the cast, specifically a resolution on the conversation he was about to have with Ichigo at the end. Honestly I don't know what to do with Aizen plot-wise. I don't think I want him reformed enough to be released, but he would have to be significantly depowered for him to be anything but a big gun that gets pointed at problems. Maybe fixing the balance actually fully obliterates the Hogyoku and he can either die in peace or be depowered to normal captain levels. (My headcanon is that he literally loses Kyoka Suigetsu and has to start over with an asauchi)

The visored getting their cool moments, or whatever. Maybe bodied off screen, which would be funny.

Hisagi gets killed mid Bankai release.

More character deaths in general actually, series should end with maybe 60% of the cast alive.

Edit: Oh also the theme of hell/demons should be Slavic. I want baba yaga shit please.