I don't think it's likely to end up being Rocks. If only because this mystery man isn't smiling. If I remember correctly, every time we've seen Rocks in flashbacks, he's had a huge grin on his face. If this guy was Rocks, I feel like Oda would have drawn him with a smile on his face.
And I doubt it'll be Shank's twin (or cousin or something), either. This time for a completely meta reason. Sure, Oda could find a way to justify Shanks having a secret twin brother this entire time. But, Oda loves designing new characters. He loves making unique silhouettes. So, I don't think he's going to have a big new character who's basically just a copy of another of his characters. I think Oda would want to design a different silhouette for him, if he were to give Shanks a secret twin. Sure, there are the Seraphim, but those are almost background fodder.
Speaking of which, I could see a Seraphim of Shanks being more likely than a secret twin, in all honesty. The Government would have access to Shank's DNA, if he was the one who visited the Gorosei. But, we've never seen the WG commission a Seraphim of someone who hasn't had a Devil Fruit before. But, we know Shanks has strong Haki, so that might be enough to make the WG want a version of Shanks that they have more control over. I think a Serpahim Shanks is possible, but maybe not likely.
To me, The Man Marked By Flames does seem to be the most likely candidate. We know he's out there. We don't know what he looks like, yet. He is someone that we're likely going to be meeting at some point of the story. I think it fits the best out of everyone we know. That really doesn't explain the sword, and it doesn't look like the mystery man has any obvious burn scars. But, of the available options, I think he's the most likely fit.
Also, here's a dark horse pick. What if this is the human form of Big News Morgans? We still don't know what he actually looks like. And I feel like Big News Morgans would have some interest in grabbing the One Piece for himself. If only for the headlines. But, we have seen that he has plently of personal ambition for himself. I think it could fit.