Early in the series development i guess. If he wrote Shanks's backstory now i doubt he would have him lose an arm to a sea king.I'm trying to figure out why a Yonkou lost his arm to a fodder Sea King.
But yeah, I'm curious to see what Shanks' fighting style is since he seems to be the only Yonko without an overpowered Devil Fruit. My guess is Oda will reveal that he has the strongest haki in the world or something.