Chances are Big Mom and Kaido will end up being our "Tour Guides to the Underworld".
Big Mom and Kaido both fell into a volcano. Volcanoes are usually connected to the molten cores of their planets. But, we already know there's something else going on with the One Piece World and it's "Underworld". The Chapter 1138 Mural seems to indicate that there was some sort of "Underground Mining Operation" going on, back during the First World. This is probably also connected to the holes in the ocean that the Ancient Weapon Uranos is creating. I think eventually we're going to visit that place and see what got left behind down there. Because chances are there's something down there that's just waiting for it's chance to return to the surface.
One way or another, I think we're going to eventually end up venturing into this "Underworld". Luffy's either going to sail straight into one of these holes in the ocean. Or Laugh Tale has some sort of express elevator to "Hades". And if and when that happens, it would kind of be useful to have a couple of characters who we already know, and have spent the last few months wandering around trying to find a way out of this place, to help us explain what's going on down here.
We've seen from Elbaph's "Underworld Layer" as well as Brook and V Nusjuro's powers, that Oda is setting up the Underworld as most likely being a cold, frozen place. Because chances are, there's something frozen in the Underworld, waiting to get thawed out. This could be the "True Sun" or "Forbidden Sun", like the original form of the Mother Flame. Or it could be a group of people, like the Dragons, who may have been imprisoned underground for the last few millennia. Or maybe both. More on that over here:
https://worstgen.alwaysdata.net/for...message-will-be-the-dragons-are-coming.55673/
Either Big Mom and Kaido will be helping the Strawhats navigate and explain this Underground world. Or, alternately, Big Mom and Kaido will end up "waking up" whatever's down there. Thus either setting off a timer for what would basically be a bomb, or leading a army to the surface to conquer the surface world. Whatever Big Mom and Kaido are doing down there COULD kick off the true final threat in the manga.
Either way, I do believe we'll end up seeing these two again. They may be enemies or they may be allies. But, either way, we're probably going to see what's "under the sea" soon enough.