I am probably late to the party and someone else has speculated about this already, but since
- the final Road Poneglyph used to be on Fishman Island
and
- we now know that the "man marked by flames" can create whirlpools
might it be that it is this guy?
https://onepiece.fandom.com/wiki/Sunbell
His name would also kind of fit his purpose, since if Luffy (the sun) manages to meet him and get the final Poneglyph, the bell would ring to announce the coming of the new dawn/the sun once Luffy and the crew find the One Piece.
Shiryuu speculates that the man marked by flames may work with the World Government but if Kuzan can switch sides (even if it's all just an act and he is part of SWORD) then why couldn't a former Roger pirate be affiliated with the World Government too.
I know it's probably not Sunbell, there's a good chance it's a new character (assuming it is not Saul) but hey that's my best guess at the moment.
Also he'd be a fried fish if he were to have burn scars on his body.