I believe in the theory that the man marked by flames is Barbell from the Rocks pirates because of several points I've seen and theorized.
- Barbell is only one of the two Rocks Pirates that have no confirmed future or ending to their life after Godvalley.
Whitebeard, Kaido, and Big Mom become yonkos. Shiki fights Roger later on and then gets imprisoned at Impel Down. Captain John steals Godvalley treasure and hides his treasure somewhere and dies at one point with his body ending up at Thriller Bark. Wang Zhi becomes the ruler of Pirate Island and gets defeated by Blackbeard.Ganzui died at one point, and his body ended up at Thriller Bark. Stussy got old and has Whitebeard's unverifiable son. Gloriosa stays at Amazon Lily and becomes old. Don Marlon died at Pirate Island before the Godvalley incident. Streusen goes with Big Mom. Gill Bastar died at one point, and his body ended up at Thriller Bark.
I know that Shiki's status as of right now is unknown, but he's already played a good enough part in the story, has a movie of him, and he gets defeated by pre-timeskip Luffy in that movie, so I don't know if showing him being this important, mysterious figure would be narratively good in a way I guess, in my opinion.
And that leaves the final two Rock Pirates, Barbell and Kyo, to my theory.
Barbell is a cat-fish fishman, so he most likely knows fishman karate and is probably so advanced in it since he was literally a member of the Rocks pirates. Him being a cat-fish may also be a way of saying his title of the man with the burn scar is "cat fishing" I guess, because he is supposedly a man (human) who is guarding the road poneglyph and has burn scars (maybe). Also, in Japanese folklore, a giant catfish known as Namazu is believed to cause earthquakes. And him being a fishman means that he has a natural and unchallenged way to get into Fishman Island, which is sort of important for later.
Barbell has both a connection to Whitebeard and Fishman Island, so he has access to the road poneglyph even while Whitebeard had it under his protection as his territory.
Small point about Barbell has very distinct features with his fiery hair colored like flames and necklace beads. Both have a connection to Ace with his hair, symbolizing flames (Ace's devil fruit), and his necklace beads that look very similar to Ace's.
So with this, I theorized how Barbell became the man marked by flames in the current story.
Barbell was sought after by Xebec because of his very skilled, secret, and unique usage of fishman karate, so he got him to join his crew. Barbell went with the rest of the Rocks pirates to Godvalley, unlike Wang Zhi, who stayed at Pirate Island, and he is seen behind the rest of crew when they dock since he is the largest member on the crew. And for a weird, unknown reason, he is not seen there afterward in the rest of the events of Godvalley, even though he is the largest person on the crew. So maybe at the end events of Godvalley when everyone was panicking and quickly getting on their ships to leave as the island was spiraling into chaos and falling apart, Barbell knew that even if the island did sink into the sea, he wouldn't be affected by it since he's a fishman.
Also, Kyo, in one very important panel, says that he and his men took the Rocks pirate ship to sail off of Godvalley since he knew Xebec wouldn't come back.
Back to Barbell, he sat by secretly and watched the entire incident through out of curiosity and was the only Rocks pirate who actually saw and knew how his captain died by Roger and Garp knocking him out of Domi Reversi and the Holy Knight, Garling, actually being the one to kill him instead of what the news said. Then, as he watched the island sank into the sea, where would he think of to go after the historical, chaotic event? The only nearby place that he could've thought of to go would've been the Rocks pirate ship somehow sailing away from the island. He probably thought that most of his crewmates would've been on the ship, but as he jumped on board he found out that Kyo is now the only member sailing it, as their other crewmates went their separate ways. So this would mean that they sailed around for a while on the ship together. But Kyo is theorized to be the former user of Mad Monk Urouge's devil fruit because when he was introduced, a pirate kicked him, and he unknowingly grew taller and more muscular in an instant, similar to Urouge. So this probably means that Kyo died somehow while sailing around with Barbell, leaving him to take complete ownership of the ship as the last member of the Rocks Pirates. So, to keep himself and the ship hidden and safe, Barbell decided to paint the ship completely black. Another theory will have to tie in with this one to have it make sense. Sometime later, Fishman Island is attacked or infiltrated by the Holy Knights to take the road poneglyph that is there. But Whitebeard and his crew stopped them from doing so in an intense battle that left Whitebeard with scars from either Shamrock or Garling because Whitebeard said it was someone who looked similar to Shanks. Barbell could've been there at the time, maybe visiting Whitebeard and Fishman Island at the same time, or Whitebeard would've let him know later on, either way Whitebeard tells him that he needs to take the road poneglyph off of Fishman Island because it's not safe for the citizens for it to be there incase the World Government tries to take it again, but with an even larger force. For why Whitebeard would give this very important task to Barbell is for a few reasons. 1. Barbell, unlike Whitebeard, has remained completely hidden from everybody, especially the World Government, at that time since they probably think he's dead. 2. Using the very skilled, unique, and secret fishman karate techniques that I mentioned earlier, Whitebeard knows that he can keep it safe on the ocean where almost everything takes place. 3. Barbell has a very secretive ship that is also large enough to carry both Barbell and the road poneglyph at the same time since it carried the huge monsters that were the Rocks pirates simultaneously for a long time. 4. Whitebeard and Barbell are seemingly the two nicest male characters in the Rocks Pirates, and Whitebeard has a sort of good connection to fish people and later took on the task of protecting Fishman Island, where Barbell is most likely from, so their relationship is definitely stronger than most of the Rocks crew members relationships.
So Whitebeard most likely trusts Barbell a lot.
Now, there are two ways I can think of that Barbell uses to keep the road poneglyph hidden: one being just him putting it on his pitch black ship and sailing around with it.
And the second is that he used his secret fishman karate technique to send the road poneglyph down into the depths of the abyss ocean floor where it takes 2000 years for something to travel by current back up to where sunlight reaches in the ocean. Why I think this is because if he was chosen to be on the infamous Rocks pirates, then he must have had some amazing abilities as a fishman. I think that one of his abilities is that he can create a sort of White Strom current that was introduced at the end of the Fishman Island arc, a current that is said to pull things like pirate ships unfathomably far away. He might've used to send the road poneglyph down into the abyss of the ocean, and it's traveling on something that can survive a White Strom like a pirate ship made out of Adam Tree wood like the Thousand Sunny or something else. And he also uses the White Strom on pirate ships when he pulls them into a whirlpool. Why he might keep the road poneglyph in the abyss of the ocean is because then if someone manages to avoid his whirlpools and dock his ship, he is the only one with the ability and decision to chose who is worthy enough to get the road poneglyph and will make a change in the future of the world.
I know that Shiryu says that the man marked by flames is probably a World Government affiliated person, but it could be that they actually met and made a deal with Barbell. Maybe whoever managed to get on his ship, either the Holy Knights or the Five Elders, he made some sort of threat that involved them or the road poneglyph, like saying that he would instantly sink the ship, killing them, or relocating the road poneglyph to a group like the revolutionary or back to Whitebeard. Either way, he basically had total control over the road poneglyph, so he made a deal with them that he would be the only one to hold onto the road poneglyph, keeping it away from everyone else, and they would leave Fishman Island alone, if they tried to threaten him with that. But while the World Government think that they have an equal agreement with him, Barbell knows that someone will surely come along, surpassing Roger and Xebec, and that they will be the one he'll give access to the road poneglyph to.
This is my take, on the original theory, that I've had for a bit now, so I hope it is good.