Oda once said that Shanks was the character that is most like himself. He is no fanfic writing teenager. Oda's self insert will not be a Mary Su character. He is self critical, he makes fun of himself. So the character portraying himself will be heavily flawed. Right now Shanks is portrayed as a brilliant guy, who is trying to hold the world together. Order is his goal, but he will fail and cause chaos. Losing against Blackbeard wouldn't be that embarrassing. The guy will have three insane Devilfruits. Everyone not named Luffy would lose. Shanks has to fail harder. My personal view is that by beating up Bartolomeo, he will unintentionally set a chain of events in motion that will cause a second Marineford war. He tried so hard to prevent the first one, he will cause the second.


