From the very beginning, Luffy made his dream of becoming the Pirate King known to the audience, and if by some extraordinary source of luck someone did forget about it, Luffy asserts that he will become the Pirate King at the end of the preview for every One Piece episode. Luffy's dream of becoming the Pirate King started all the way back in Foosha Village when he met Shanks.
Shanks inspired Luffy, and the latter decided to become the Pirate King, just like Gol D. Roger. For a long time, Roger's past remained a mystery, but Oden's flashback provided the fans with just enough to form an opinion about him. Gol D. Roger lived 24 years prior to the beginning of One Piece, and at the time of his execution, he was the most famous man in the entire world. He left behind such a great legacy that it is almost impossible to supplant it.
Setting Sail And Conquering The World
77 years before the series started, a child was born in Loguetown, and at that time, nobody would have expected that he would end up becoming one of the most legendary pirates in history. Roger met Rayleigh sometime during his youth, and their meeting eventually led to the creation of the most notorious pirate crew to ever set sail. Around 39 years ago, Roger and his crew arrived at Lodestar Island, which they assumed was the final island, but they were proven wrong.
After finding out that he did not have much time left, Roger decided to try his luck again after 11 years. This time Roger begged Whitebeard to allow Kozuki Oden to travel with him. With Oden by his side, Roger finally had someone who could read the Poneglyphs and point him toward the Laugh Tale. Roger's decision to bring Oden aboard proved to be fruitful as the Roger Pirates managed to set foot on Laugh Tale and found out about the true history of the world.
After finding the treasure, Roger disbanded the crew and turned himself in to the Marines. At the time of his execution, Roger was still in high spirits. When asked by a random person about his treasure, Roger confirmed that he left his treasure behind, and it was there for the taking. Roger's last words effectively started the Great Age of Pirates.
The Heir
Roger left behind an incredible legacy, and all the pirates that have tried to replicate it have failed. Whitebeard was a legendary pirate, but not even he managed to shake things up like his rival, Roger. However, as fate would have it, another person would be born five years after Roger's execution, and he would become one of the most infamous pirates in the world, and that person was none other than Monkey D. Luffy.
When Shanks met Luffy, he was impressed by the kid's determination, but what moved him more was Luffy's promise to become a great pirate, and uttering the same words as the late Pirate King. So, when Shanks saw Luffy on the cusp of death, he had absolutely no hesitation in sacrificing his left arm. Shanks's sacrifice even surprised Whitebeard, and when the latter inquired about who was irresponsible, Shanks simply said he bet it on the new era.
The Journey To Become The Freest
Luffy has gone through numerous ordeals and no matter how dire the situation, he has always managed to pull through. Currently, Luffy is among the strongest characters in the world, but it is not power that he is after. Luffy wants to become the freest person in the world- a dream that he shares with Roger. Often mistaken for being immature, in actuality, Luffy is very smart and understanding.
Luffy knows his responsibilities as the captain of a crew and how his crew depends on him when they are stuck in a bad situation. Luffy has worked hard over the years to make sure that he is powerful enough to protect his crew. Due to Luffy's nature, his crewmates are extremely loyal to him, and they have absolutely no hesitation in laying down their lives for the sake of their captain.
Joy Boy
When Roger visited the Laugh Tale, he claimed that they were too early, so they decided to leave the One Piece untouched. It is fair to say that Roger was not the person, who would change the entire course of history. However, he did start the Great Age of Pirates, which all but ensured that the person destined to change the world would come forth. Luffy has already shown that he doesn't fear death, and he is willing to accept it with a smile on his face. With his actions, Luffy makes friends everywhere that he goes. Dracule Mihawk acknowledged that Luffy's ability to get people on his side was a truly frightening power.
In addition to this, Luffy also possesses the Hito Hito no Mi, Model: Nika, a mythical zoan type devil fruit. When Luffy was knocked out by Kaido, the devil fruit started taking over, and his persona changed. Zunesha, who was talking to Momonosuke confirmed that Joy Boy had returned. It is evident that the only person capable of surpassing Roger is Luffy. Even though Luffy still has a lot to achieve, he is the only person who can transform the entire world.