Lets see, the core concept here is "With great power comes great responsibility", when Luffy reaches Raftel (and be crazy strong) and learn the true history, he would undoubtedly come to realize that there is still stuff that needs to be corrected and that he, as the Pirate King, is now responsible to do that. You can't be free when you have responsibility, therefore in order to create a world where others are free, he would have to fight the greatest evil (maybe WG, maybe revolutionaries). At least thats how I imagined it.
Roger became ill and couldn't do that himself when he finally became strong enough, so he urges others to try and reach the end of grandline to find someone (remember Crocus and WB saying that Roger was waiting for someone?) strong enough to finish what he couldn't and thats where Luffy comes in.
The series focus has always been that to a noble man, treasure is not gold or materialistically valuable things, but things which one holds dear. To Roger; who could hear the voice of everything, who created the great age of piracy, and who is described to hold a similar personality to Luffy; doesn't something as basic as the philosophical truth regarding freedom seem fitting to be the greatest treasure worth risking your life for?
Plz Rogers didn't care about the wellfare of the world he was a pirate. He wanted to make a new pirate world in his image. Just like Luffy doesn't care. By the time he's PK he'll be considered the greatest evil like Rogers and WhiteBeard were. Any world war won't determine peace because so far the biggest threat against peace is pirates just like Luffy.
You see how many people were killed and towns looted after the war? Thats all pirates. You think Luffy is going to stop fightings or stop misbehavior between the revolutionaries the marines and pirates?
@Xajen:
I hope Oda doesn't do something stupid like they get to Raftel, Luffy realizes the treasure is not there, he turns around and looks at his crew and smiles realizing the adventure and his friends are One Piece, or something. That would be so lame and far too moral.
Roger said if you want my treasure head to that place. If it's on raftel than it's not a hat or something. Luffy said to him his hat is a treasure but he never mixes up gold with his hat.