To summarize Shanks is a mysterious, powerful character. He seems to be able to have influence that I believe goes out of the scope of his Yonko status, and I believe it’s because of a Celstial Dragon lineage. However, there have also been hints at another connection to his which I believe tie to his father Rocks.
The Scandal & God Valley
If Shanks is the son of Rocks and a Celestial Dragon then there must be a scandal.
Here is my made-up backstory
Xebec is a slave to a very prominent Celestial Dragon family. They all treat him harshly except the youngest daughter who is extra kind. She gives him extra food, she helps educate him, and ultimately they fall in love. Upon discovery of their romance Rocks is set to be killed until he is helped to flee from Mary Geoise by his love. Because they were not able to be together Rocks sets on a mission to throw the world in chaos and destroy the Celestial Dragons altogether. He rises as a prominent pirate, is somehow able to continue a romance with his love and she is found with child. Upon this discovery, Rocks’ love is sent to God Valley where he launches an assault some time later and this leads to chaos, death, and destruction. He dies along with his love but Shanks remains alive.
Why Shanks is given to Roger
Because of the notorious affair and them wishing to erase all relations to the scandal, it is deemed best that Shanks is given to Roger. This decision was made by Garp. Garp fears that Shanks will be killed by the Celestial Dragons because of who his dad is. This is where he says the same words that Roger utters to him some years later on request of taking care of Ace, A child doesn’t bear the sin of his parents. Parents. Plural, as it is not only referring to Rocks as a man, but the action between both of Shanks’ parents that lead to his birth.
Roger reluctantly agrees but ends up being fond of Shanks so much so that he passes his will unto him in the form of the “Straw Hat.” (I have theories on how symbolic the straw hat is in relation to the Void Century, and the will of D, but that is another post). Roger tells Shanks some time later about his heritage before they reach Laugh Tale and that is why he cries.
Why isn’t Shanks the Pirate King?
With a backstory like that you would think Shanks would be a perfect fit to overthrow the Celestial Dragons. I think the reason he doesn’t become the main force and instead transfers Roger’s will to Luffy (by passing down the Straw Hat), is because he wants to avoid the path of his father. This is similar to Ace also rejecting the will of his dad.
The fate of Shanks
Again, Shanks is a man of balance. Right now there is a pirate on the path of chaos, a pirate that has inherited his father’s will. That pirate is Blackbeard D. Teach. Shanks will oppose Blackbeard near the end of the manga and die.
His death shifts the tides of balance and it is now only 2 parents with the Will of D. who decide the fate of the world. Blackbeard who fights for chaos, and Luffy who fights for Freedom.
Shortly after Shanks’ death, Luffy arrives to place the straw hat on Shanks’ lying corpse thus fulfilling his promise of returning it when he becomes a great pirate. Luffy and Blackbeard have the last showdown where the winner makes it to Laugh Tale.
*Theory by KillaKast16