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.