One Piece is inspired from Vicki The Viking which also inspired Movie Trilogy "How To Train Your Dragon"
All these Three Series have Two Characters in common
A Weak Kid Viking Warrior who saves the day using his Intellect & Guts + Strongest Warrior who rules with a Mighty Fist
In Vicki The Viking, The Kid is Vicki himself, while Strongest Warrior is Leader with Eye-Patch & Red-Beard
Which is similar to Hiccup from HTTYD & his Father (Red-Bearded but no Eye-Patch though)
In One Piece, both Characters exist
The Kid is Original Nika (His Ability is Power of Imagination, cuz He relied on his Intellect & Tactics & Guts) & He was from Elbaf (Vikings).
Ruler of Elbaf at that time is also known as their God of War was Red-Beard & He had an Eye-Patch or at least was One-Eyed
Luffy is not like Nika, because He isn't a Weak Character who mostly relies on his Intellect to Win due to lacking Power.
However, He did inherit a DF Manifestation of Nika granting him Super Powers that makes him similar to Nika. So who is New Nika?
It's obviously Usopp, He is the one who is Portrayed as a Weakling, relies on his Intellect & wants to be a Brave Warrior like Giants (Basically Vikings)
In other words, Original Nika have Two Inheritors, Someone who Inherited his Character (Usopp) & Someone who Inherited his Will (Luffy)
It's like Usopp's Arc is a Re-imagining of Original Nika's Story, while Luffy is his Successor (Whose DF also made him resemble Nika Physically)
The same applies to Second Viking who is One-Eyed & known as Original God of War (OP's Version of Red-Beard)
Someone Inherited his Character & that is Zoro (Hence why He is One-Eyed, Crew's Fighter, have Top Warrior Qualities, Dreams of being Strongest ... etc) while Someone else actually Inherited Powers of that God of War & that is Final Villain
Basically Nika & Red-Beard were World's Strongest but One is all about Brains & other is about Brawns
Usopp & Zoro are basically Fate repeating itself & are a Re-Creation of the Story that made Nika/Red-Beard known as Strongest
However, Luffy & Final Villain are the ones who actually Inherited the Wills of those Two & represent a Continuation of their Story where they are Fated to eventually Fight at the End (Their Final Forms are gonna be Strongest Ever, similar to Shadow Asgard Moria & Nightmare Luffy in Thriller Bark)
Since Nika is all about Imagination while Red-Beard is all about Overwhelming Power (Similar to how Usopp tries to Outsmart his Opponent while Zoro simply tries to Defeat them in a Direct Clash), Nika's Will represents a Free World (Hence why He is Warrior of Liberation) while Red-Beard's Will is about a Strict World (Hence why he is Final Villain)
Luffy & Final Villain are fated to Fight because it's gonna be about "Will To Create Free World" vs "Will To Create Enslaved World"
(Which is Theme of almost every single Arc)
So i'm explaining all of this, to say that Man Marked With Flames could indeed be Final Villain (New Character but Mentioned before) & He could very likely be the Eye-Patched Character Oda talked about