So those of us who are reading the Manga are aware that Kaidou and Big Mom were "defeated" on Wano. Both of them knocked deep into the earth. Is there a chance both of them died? Yes. But these are two characters that have shown the absolute toughest durability so far in the series. Kaidou's hobby is attempting suicide.

It was also revealed that Pluton is 'sleeping' beneath Wano.

So, the theory doesn't need a genius to put it together, and has already likely been covered, but do we think Kaido and Big Mom are going to return when the borders of Wano are knocked down, the water drains, and presumably Pluton is unleashed on the world?

Personally, I would say yes, and am going to continue assuming that I am correct. If you dislike this idea you can stop reading thanks!

Now, Kaidou is an awful, vile person who caused the suffering of thousands, however during the rooftop fight, and particular when he was fighting Luffy by himself he seemed to greatly enjoy the combat, not caring for different ideals.

Big Mom is an equally awful, vile person who steals people's souls to give them safety. However during Wano she was given specific reasons to like the straw hats (helping her friend Otama, amnesia travel with some of them.).

I understand that the idea of Big Mom and Kaidou returningas allies in a larger conflict is fan fiction-y, however I genuinely do believe that when the borders of Wano are opened in the Final Saga, it will be with Big Mom, and Kaidou riding with the greatest Shogun in Wano's history aboard Pluton (or inside if it's a living creature) to war Against the world government (assuming that pudding is taken prisoner by the marines on Hachinoso).

What kind of work would require two former, near indestructible beings? Perhaps the piloting of Pluton itself. This is supposedly a weapon that can destroy islands, it may very well require these two working together to direct Pluton to its final location. (If not this, they will likely just chump a swarm of vice admirals.)