First of all this is not my theory but I’m just bringing all the interesting points I saw throughout Internet together which would give you all an overall idea about how certain it is for Twin Shanks to join the story.
It all begun from the Reverie arc when we got to see a guy who looks exactly like Shanks at first glance visit Gorosei to talk about a certain pirate. Oda only drew the other side of his face very well hiding his scar and the missing hand through the cape and even in the anime which supposedly is known for making the pacing ugly by giving multiple camera shots of same character very carefully only showed us one side of Shanks exactly like Oda as if they were trying to hide the scars and the missing hand because in every single other chapter Shanks has been in, there’s always the scar drawn neatly with the focus but this one single panel which created a discourse and multiple allegations on Shanks didn’t which is weird.
Second point is about the speech he used while confronting Gorosei which was formal. This point is extremely worth noting because Shanks akin Luffy has always been showed using informal Japanese as a mean to communicate but at this one panel he is different. One can argue that it is because of the status the Gorosei is on but then that means Shanks should have also used formal japanese while talking to someone like Whitebeard or Sengoku but he didn’t which is very weird.
Next point, is the man with the burn scar who holds the last Poneglyph. We are shown him getting mentioned multiple times since Wano and most of the assumptions are Saul but in my honest opinion it is none other than Twin Shanks who took the Poneglyph from Fishman Island and gave a scar to Whitebeard because the last known location of that poneglyph was in Fishman Island which happens to be Whitebeard’s place thus it only makes sense for the Government to send their knight to that place so that no one reach the One Piece and even Blackbeard theorizes that the last stone must be with Government.
One common misconception about the scar of Whitebeard in this sub is that it’s given by Roger before his death or Garling after God valley which is wrong because we have already seen Whitbeard during Oden’s backstory which takes place after God Valley and showed the last duel between Roger and Whitebeard thus debunking any such claims about the scar being given by Roger or Garling as he had nothing on the middle of the chest but he did got one after sometime in New World and in my guess it is by the Twin Shanks.
It’s also worth noting how Whitebeard made a comment about Shanks’s face and related it with the scar.
My last point is about Kid, he is shown attacking Shanks and talks about how he will snatch the Poneglyph. In my opinion, he attacked him because he got some intel regarding the guy who holds the last Poneglyph but due to miscommunication perhaps the guy who told him the information got baited due to the similarity of the face of shanks and his twin and thus told Eustass to take on Shanks.
*by blueontheradio
