* Gerard vs Shinji, Hiyori, Love, Lisa & Hachi without the others. Hachi should've frozen or bound Gerard long enough for the others to get past. Zaraki's bankai could be moved to an extended version of the Pernida fight alongside Mayuri. As much as I love the latter he's had way too much screentime and it's unfair for everybody else. Toshiro & Byakuya, along with Renji and Rukia, can have their battles moved to the end of the arc against Yhwach... One other thing; I'd personally change it so that Pernida was actually the black-eyed woman Jugram spoke to in Wandenreich and the "left arm" was being controlled through The Compulsory. That way Zaraki can go ham on the arm and its cancerous proliferation whilst Mayuri tries to outdo Pernida's Compulsory.
* Mask vs. Hisagi, Mashiro and Zombie-Kira. The Kira reveal creates a nice bridge between this fight and the Pepe/Giselle fights that come almost immediately afterwards (zombie Rose, corpse squad, Mayuri etc.).
* Nodt vs. Kuukaku & Ganju, stepping in to fight alongside Rukia. We can also use this as an opportunity to delve into their fear of loss and how it's affected all of their psyches. It can be a nice little exposition dump for the Shibas, more than anything.
* Liltotto, Meninas & Giselle vs. Ukitake, Isane, Sentomaru & Kiyone whilst Ichigo is fighting Candice. Isane shows up first to protect Zaraki from the femritters, then the other three show up and commend her on her courage and determination. Typical Ukitake stuff "that's what it means to be a Gotei officer" etc. You could even have Mimihagi "stir" and freak out the femritters before the official reveal.
* Shaz vs. Isshin and Ryuuken as the perfect punching bag. Ryuken talking mad shit about what the quincys have become whilst Isshin picks his ears out of boredom would make me laugh so hard.
* Iba, Ikkaku, Yumichika & doggo Komamura vs. BG9, because killing both it and Cang Du makes zero sense. If K is The Knowledge, it'd be interesting if BG9 could read information from reiatsu, thus "knowing" everything about its opponents and revealing their secrets. Ikkaku and Yumichika wouldn't be able to stay in the metaphorical closet anymore. Plus we can get the name of Iba's shikai, even if it's BG9 who mentions it.
* Ginjo, Tsukishima & Giriko vs. Cang Du. This and the BG9 fight can occur after Zaraki is defeated and all the sternritter enter the fray like wild dogs.
* Chad, Orihime & Neliel vs. Nanana & Robert.
* Grimmjow & Harribel vs. Yhwach (Part 1). This could be the moment where he uses Yamamoto's stolen bankai to resurrect the dead and toy with the arrancar whilst he waits for Ichigo to arrive. The other Shinigami (Omaeda, Suifeng, Byakuya, Toshiro & Matsumoto) could also appear soon after as an excuse to show off their abilities. This battle itself could become a meeting point for Isshin, Ryuken, Yukio, Riruka and many others before Ichigo shows up and Yhwach utterly humiliates him.
* Final Battle, like I mentioned earlier, could include the fan favourites alongside Aizen and Ichigo.
Edit: As for the Zero Division, you could have Ichibei call their names and bring them back so they can join the everybody vs. Yhwach penultimate battle.