Thank God the man with the Burn Scar isn't Saul.....
Only option at this point is it is a former Roger Pirate member. Who else would be riding around with a Road Poneglyph, but not trying to find the others for themselves? Someone like that would be a big name in the New World if they were an active Pirate, and it seems like this dude is currently only known as "The man with the burn scar.", so he's not well known.