Orochi have Two Heads Remaining i think
I believe Zoro will meet Hiyori & then both meet Orochi & Zoro kills him & cost his remaining Heads
Hiyori will treat Zoro's Wounds while others are Fighting
Meanwhile, Luffy is doing great against Kaido but eventually loses again or his CoC runs out (If his CoA runs out then why not CoC too)
That is when Yamato & Momo arrive & help Luffy
Zoro after being healed, decides to go up again & check on Luffy with Hiyori
I don't know how things will play out there but these Characters should all be on the Roof:
Kaido, Orochi, Luffy, Zoro, Momo, Hiyori, Yamato
Since Momo & Hiyori shouldn't meet until Kaido & Orochi are both taken down then i imagine once Hiyori goes up, Momo will be flying,
Here is imo the Ranking of Events:
01. Luffy vs Kaido 1 v 1
02. Zoro meets Hiyori
03. Zoro & Hiyori meet Orochi & remove his Remaining Heads
04. Luffy either Runs out of CoC or is again Defeated by Kaido (Very Close Fight)
05. Kaido is going to Kill Luffy but Yamato & Momo intervene
06. Momo (+Shinobu maybe) carry Exhausted Luffy out of the way
07. Yamato vs Kaido (Mostly Drama & Family Talk)
08. Kaido Defeats Yamato
09. Momo gets courageous & challenge Kaido to a Fight
10. Momo turns into a Dragon & flies to the Sky
11. Kaido laughs & follows him in Dragon Form too
12. Zoro & Hiyori arrive to find Kaido chasing Momo & Yamato/Luffy on the ground
13. Luffy & Zoro are thrown to the Sky (By Yamato most probably or Luffy uses Gear 4)
14. Human Orochi arrives & takes Hiyori Hostage while at same time Kaido reached Momo
15. In Same Chapter, Yamato defeats Orochi in Oden's Name & saves Hiyori
16. While at same time, Luffy & Zoro do their Ultimate Combo Attack & take down Kaido (Most probably gets beheaded by Zoro)
17. Momo carries Luffy & Zoro who just gave everything in this Last Combo & fly them to the ground
18. Momo & Hiyori finally meet after both Kaido & Orochi got Defeated
As for Onigashima, there are two possibilities:
1. Momo will Unlock his DF Powers & create Clouds to prevent Onigashima from falling after Kaido removes his own Clouds which is possibly how Momo made Kaido chase him to the Sky
2. Big Mom instead of being Knocked Out, will turn to O-Lin again after being possibly Defeated by Kid & then she decides to Stop Onigashima by making those Clouds Homies or some other Ability to mirror Kaido's Feat
Because i don't think Onigashima will be Destroyed, cuz the Rubble will still destroy Wano, that's why someone must stop it & only BM & Momo have Abilities to keep Onigashima floating & put it somewhere else