Roger knew that he was going to die, so he made sure that his death wasn’t in vain but actually was the first step in their big plan.
Finding the right person
The next step in this plan, is finding the right person among all the candidates. The person who was likely responsible for this was Shanks. We know that Shanks founded his own pirate crew and rose to power and become a well-known pirate. Shanks travelled the world and saw many places and likely met a lot of people. His journey into East Blue could have been for the purpose of finding Roger’s successor. When he met Luffy, it quickly became clear that he had the willpower, and the spirit that Roger also possessed. Members of the Roger Pirates have even expressed that they see the same spirit in Luffy that they saw in Roger. Shanks ultimately chose Luffy to be the chosen person to complete Roger’s mission.
It is possible that Shanks his Straw Hat isn’t just a token that ties Luffy to the title of Pirate King, but it is also a sign to other members of the Roger Pirates that he is the chosen person. Some of the Roger Pirates Crew members have possibly set up post along the Grand Line on points that all the big time pirates will pass (Sabaody) in order to guide the possible successor. The first person to do this was Crocus at the entrance of the Grand Line, and later they met Rayleigh at the end of paradise at Sabaody. Rayleigh trained Luffy in Haki during the two year timeskip and assisted Luffy in preparing for the New World. Because of this, Rayleigh is responsible for helping Luffy another step further in his journey to the Pirate King title and the final island Raftel. It is possible that more members of the Roger Pirates will show up before reaching Laugh Tale, like Scopper Gaban.
3. Purpose of the mission
It is possible that the Rio Poneglyph contains information about the World Government that turns the world upside down and cause a massive war against the Celestial Dragons and their governing organisation.
The Pirate King could become the leader against the ruling forces and because Roger was terminally ill, he couldn’t start this himself but instead decided to devote his crew to finding a person and pirate crew who would be able to do it instead.
Luffy and the rest of the Straw Hats could ultimately be the people who find the truth and unite various factions from around the world against the corrupt and possibly evil organisation that we know as the World Government.
*Theory by S.