Even if Kaidou and Big Mom return they won't even pose a threat to Sun God Luffy and Sword God Zoro.
In my opinion, this mirrors Mr. 5 and Miss Valentine in Whiskey Peak. If Luffy and Zoro are supposed to be fighting each other at some point in Wano, then there has to be a reason for it to happen. The simplest reason could be Zoro literally turning evil. If you think about it, Kaidou, Big Mom, Green Bull and Luffy all seem good based solely off their goals. Zoro becoming evil and soloing them all has been hinted at in Wano so far with Zoro beating Hawkins, Killer, Apoo and King.
When Zoro blocked Hakai, I think he also "stole" how to perform Hakai himself, like with Kin'emon's technique. Hakai was performed with just two people, but isn't Zoro's Asura form essentially three people? We've yet to see a flying technique from Asura form, and now that we know Asura is ACoC wouldn't it mean a ranged Asura attack would be stronger than Hakai?
My theory about Zoro and the Sword God, is based off Link from Zelda and how he's the same hero, but different. I think Zoro inherits something from each past Sword God like with Ryuma's eye. There's also those earrings from the moon drawing, Zoro had those the earliest, there's also his bandanna. His three sword style which I think is his own contribution to the Sword God.
(Kaido rolls out of the volcano)
"Awww, fuck, what time is it? Awww, fucking hangover, shit. Where's the sake, I need to drink this off."
"Hoooh, shit, what happened to my horn? Damn, this year's Golden Kagura must have been a real rager, did Linlin do this? Hope Onigashmia is okay, fuck. Can't believe I woke up in a volcano."
(drinks some lava to sober up)
"Wait, fuck, shit, we lost. Damn. Can't believe that Straw Hat kid is Joy Boy, what a crazy world."
(Suddenly stands up and starts shouting)
"Oh, shit, fuck, shit, I forgot, Zoro is supposed to kill me, fuck, where are my keys? Fuck, I'm way too drunk to drive, even after a week with no booze. Fuck, I don't even drive, I fly, I'm a fucking dragon, shit."
(Kaido crashes into the side of the volcano, and notices a newspaper falling into it. Kaido reads it quickly)
"Ohhhhh, fuck, it's been a whole week, I just ruined the whole damn manga! One month break!?! Shit, Oda had to take a month off, because my drunk dragon ass forgot about ZKK, damn it. I can't do anything right, fuck."
"Sorry, Zoro, brother, I'm coming! I need an intervention after I get decapitated, damn it."