Hey allIn every arc in which luffy fights a boss or the straw hats fight an organization, the story goes like:
- There is an evil man
- He forces the citizens to obay him or wants to kill them all
- The citizens hate him and want someone to defeat him
- Luffy shows up and defeat him
- Luffy become a hero the citizens love him
That happend in Arlong park, Alabasta, Skypiea, Fishman island and Dressrosa.
Depending on that, i think that the grand battle that will happen on Raftel (or near it) will be like that but on the scale of the world, i mean that the citizens this time won't be the citizens of a small island, they will be the citizens of the world of one piece, someone will be very strong to stop and will be a big threat on the world, and after Luffy defeats him, Lutty will be a hero and every one will love him, and It seems logical because Luffy will change the consept of being a pirate, and olso Luffy always believed that the pirate must be good and be loved by people and not evil.
The question now who will be that one luffy will defeat ??? I say he is BLACKBEARD, there are many reasons, first because he is the one who caused the death of ِAce so Luffy will revenge from him in a grand battle, second he looks really powerful, we still hadn't seen him fight with the two frutes and he might get the third and the fourth!! olso when he met Luffy in Impel down they didn't fight so they must have their fight later, i think he and his crew will get very strong, and even the other yonko will not be able to stop him and the admirals and Sakazuki will not be able to do and he will be a big threat on the world, and finally Luffy defeats him and be the pirate king.
Anyway, after that, Robin will read the Rio poneglyph to know the real history, and the thing that world government was trying to hide, they will know that the world government is bad and Luffy decide to make one last fight against the world government and almost everybody will join in the fight, and Luffy will kill Sakazuki to revenge to Ace, and after the world government is down, LUFFY WILL BE THE KING OF THE WORLD, and the series ends.
Olso between those battles the crew will achieve their dreems, Zoro will defeat Mihawk and be the strongest swordman, Brook will meet laboon, Nami will draw a world map, ... etc.
Maybe the last part about the world government is wrong but i am sure that Blackbeard part is right, and anyway it is just a theory,