I'd say it's one of two possibilities. Either Ancient Giants, or Dragons, who found a pathway to the surface from the Underworld.
Mind you, to me, that's kind of the same thing. Because, I have theories that say that Ancient Giants, and thus regular-Giants, are basically the result of Human-Dragon relations. Ancient Giants are hybrids of Humans and Dragons. Thus explaining why Ancient Giants have horns, in the first place. Like how Wotans are hybrids of Giants and Fishmen. Why Dragons, specifically? More on all that over here:
https://worstgen.alwaysdata.net/for...message-will-be-the-dragons-are-coming.55673/
But, you know what? Let me explain why I think the mist means that these creatures come from the Underworld.
First, let me explain what I mean by "Underworld". I'm talking about LITERALLY underground. I think the One Piece World is likely hollow. And, yes, there probably ALSO is some connection between spirits and souls and this Underworld. Because Brook's probably already been there and back, before. He's told us that himself.
And I think there are some pretty solid hints that there is some sort of literal Underworld. That's really the only way the holes in the ocean make any sense, to me. Plus, according the Chapter 1138 Mural, there was some sort of Underworld Mining Operation, where slaves gathered "Sparks" for their masters. These "Sparks" are most likely some sort of "Dream Energy" that is used to create Devil Fruits. Like how we saw SAD chemicals be used to make SMILE Fruits. Chances are, this Underground Mining led to the planet becoming hollow over time.
So, what about the mist? Well, creating holes in the ocean seems to also lead to the creation of mist and fog being released, like we saw when God Valley collapsed.
Fog and mist that also carry diseases, like what we saw with Brook's flashback.
That's because this fog and mist are likely caused by the decayed bodies of Dragons that are buried underground, in the center of the planet.
We've seen with Momo and Kaido's Dragon Devil Fruits, that Dragons have a innate ability to make solid clouds. So, if a Dragon, or if MANY Dragons, were to die, it stands to reason that their bodies would release these cloud-making chemicals from their body as they decay.
This explains Pyrobloin. Pyrobloin, which is said to come from inside the planet, usually from volcanoes. The same substance that we know helps make up Island Clouds and Poneglyph stone. I think when Pyrobloin combines with water, it becomes harmless Island Clouds. And if Pyrobloin comes in contact with rock, that rock becomes harder than steel, thus explaining the Ponglyphs. But, if it comes in contact with something biological, the "hardening" effect of the Pyrobloin can become quite deadly. This possibly explains what Sapphire Scale actually is. "Blue scales" like Kaido's true-blue-dragon form. A unnatural hardening of the body, caused by exposure to Pyrobloin, released by dying Dragons from Underground. So, simple, really. (Yes, I'm kidding, I know this is a weird theory! But, it WORKS!)
There's also some real-world reasons for there to be Dragons underground. It all ties into Feng Shui, where it's said that there are literal Dragons underground, moving energy under the planet. It's their version of ley lines, basically. "Dragon Holes" and "Dragon Veins" are important parts of moving and directing this energy in beneficial ways. It all comes back to Dragons.
https://www.fengshui.net/allied-interests/the-mysterious-underground-energies-8167/
https://www.easytourchina.com/fact-v1741-chinese-dragon-in-feng-shui
https://360nomad.org/dragon-holes-of-hong-kong/
I haven't even touched on the fact that this Underworld is likely also frozen. Due to the hints we've gotten with Brook and V. Nusjuro's ice attacks tying into the concept of frozen underworld Naraka from Buddhism. Plus, ice is just super-effective on Dragons, everyone knows that! Plus, we've already seen the legendary Gallelia Giants frozen back on Punk Hazard.
https://en.wikipedia.org/wiki/Naraka_in_Buddhism
Or the fact that the leftover "waste rock" from this Underworld Mining Operation is probably also what the Moon is made of. Thus, possibly explaining why Nika is associated with the Moon. That's not a Moon. That's the original center of the planet, where the Forbidden Sun used to be, before it all got mined away. The leftover rock from mining the center of the planet got chucked out into space, where over time it formed the Moon as we know it.
Or that, recently Imu has mentioned that "This battle shall divide the world" (Chapter 1177) and I think that should be taken rather literally. The world's about to be split open. Like a giant Pokeball. Or a giant waritama confetti ball!
But, I think I've covered enough to chew on, for now.
Some might call this theory, or series of theories, "over-complicated and contrived". And they would be right. But, I just have a feeling that something along these lines is going to turn out to be true. This manga's been going on for almost 30 years. The explanation for how this world works probably won't be super-simple.
Those mysterious figures in the Florian Triangle are probably the Dragons who have managed to survive being frozen in the center of the planet for the past 800 to 3000 years. Thawed out by the planet's outer shell collapsing and unthawing them. Ready to get their revenge on mankind for imprisoning them for so long. For better or for worse, these guys are my bet for the True Final Threat of One Piece. The Dragons ARE coming, mark my words!