Reason I think Shanks' brother, Blaze, is the Man Marked by Flames:
-I believe it has to be a new character, that is powerful, who has been introduced recently. Based on portrayal, it doesn't make sense for someone weaker we already know of to be this character.
-Shanks is scarred by a blade (hence, "Shanks"). Shanks' brother also being scarred makes sense, in this case, with fire (hence blaze).
-Both are tied to the sea. Shanks was introduced saving Luffy from a Sea Beast. How was Luffy nearly killed in the opening of One Piece? A whirlpool.
-The Fish-Man Island Poneglyph could have easily been stolen if Blaze is a Celestial Dragon with a Whirlpool Fruit. We know from Myosgard that the Celestials DO have the knowledge to get to Fish-Man Island, and Mary Geoise is not far away from it, just vertically high up. Easy to steal.
-This explains why no one has been able to get the Poneglyph: the person guarding it is near Pirate King level.
-Kidd and Law were the two characters who reacted to the Man Marked by Flame. Kidd was beaten by Shanks, and Law was beaten by... the guy who scarred Shanks, Teach, who everyone thinks will fight Shanks.
-Shanks was the greatest reason Luffy became a pirate. Luffy's greatest obstacle being related to Shanks makes sense.
-Imu IMO represents Big Mom on a lot of levels. Won't let people leave the family; obsessed with controlling family; giant spy network; acts childish; destroys anyone who disobeys.
-Blaze has been hated his entire life, because of Shanks. The more famous Shanks became, the more Blaze was hated by his family for looking like a famous pirate. Shanks eventually became a Yonkou, and Blaze was fully exiled.
-Knowing Shanks wants to fulfill Roger's legacy, Blaze controls the last Road Poneglyph, and desires to kill anyone associated with Shanks, especially his brother himself.