The Uniqueness of the four Emperors
Blackbeard is known for his weird body which may potentially carry 2 or more devil fruits.
Kaido is known for being the strongest creature in the world however what really ends up defining is its “immortality “ if we may say.
Big Mom is known for her hunger that makes it devour entire cities.
When it comes to Shanks it is important to understand that if a character like him that does not have an arm surely just godly haki powers wouldn’t be enough to describe him, and to justify the reason as to why he became an Emperor of the sea in the first place. Of course, the theory of him being a Celestial Dragon may change some things as to hidden powers that he might have but let’s take that out of the way for an instant.
It could be a new level of haki never seen or that he has a godly use of Haki and that he has mastered all of the possible haki combinations, but I think that that would be way too vague, for a Yonkou, because if Kaido is considered the strongest creature in the world of One Piece, surely there has to be something the defines Shanks as a character that was capable enough to stop Kaido from joining the Marineford War right?!
I wouldn’t think that Oda would give him Overpowered Haki powers just to justify the fact that he does not have an arm (even though we know that he willingly gave his arm away to trust the new generation which is Luffy). Also, this power may also justify the reason why everyone stopped when he stated that he would fight anyone that would want to step forward against him at the war. Yes, you can probably say that at that time most of them were exhausted, but is it really just that though? Surely there needs to be something that makes them fear him at a greater scale.
For this reason, my theory is that the uniqueness of Shanks is that if he shows his true powers he cannot be stopped!
I know that this might sound crazy, but stay with me on this one. I went further into his origins and into the inspirations of his character, Shanks was inspired by the Vikings and Nordic culture and themes which makes him strongly connected with Elbaf.
So now that we have the foundation this leads us to the second part of the theory which is, what is the principal characteristic of the power that Vikings portray? What is their unique ability? I would say “Berserk Mode”.
The most probable explanation for “going berserk” comes from psychiatry. The theory is that the groups of warriors, through ritual processes carried out before a battle (such as biting the edges of their shields), went into a self-induced hypnotic trance. In other words to suit the world of One Piece with intense devotion the user of such ability manages to release all of their inner anger or all of their spirit and enter a state of frenzy, thus increasing all of their capabilities allowing them to destroy everything around them.
However, in order to use this skill the candidate will have to pay a very high price, the candidate must abandon his sanity and logic getting reduced mental capacities when he enters such a mode. This may explain why we’ve never seen Shanks in a serious fight and why wars stop when he arrives, because if he gets to that state or if someone hurts his friends and family he can’t be stopped and people know about it. This may be similar to Big Mom’s rage, but Shanks’ rage reaches unimaginable levels, it’s possible that this Berserk Mode causes his Haki to overflow to unparalleled levels too!
In the Manga, it is stated that Shanks may become a demon if someone messes with his friends/relatives/family, this may be the reason why this is said, because of Berserk Mode.
Now, you may ask about Mihawk, probably, I thought that since Shanks had lost an arm, and Mihawk was considered possibly the strongest swordsman in One Piece. Why couldn’t Mihawk become a Yonkou instead since he could easily beat Shanks (common sence) he even said that it was pointless for him to fight against an armless swordsman/pirate. I believe that it is that uniqueness of Shanks that sets him apart from Mihawk, despite his political influence, and longevity and knowledge as a Pirate or even connections with other affiliates, such as the 5 elders or Mariejois.
I felt like that there needed to be something that would distance himself from any other Pirate justifying his position as a Yonkou and why he is so feared by most, I believe that this could be the reason behind it because this power would match his character theme. This is why Kaido rates him as on of the few individuals that can match against him.
I know that it was quite long read but feel free to drop your ideas below.
Theory by RheoKai (https://www.reddit.com/user/PuzzleheadedBread726)