One Piece is about nice guys and evil people and reasons for the nice guys to hit the evil peoples.
Oda liked the idea of Pirates, the idea of people weak or strong fighting for their community and the idea of a lot of races with all races being equal in rights and the idea of Freedom.
Luffy gather all that Oda likes.
A carefree, genuine, gentle Spirit with an inflexible determination.
A little like Roger, Zoro, Oden and Shanks in their own way.
Here they are : the basic forms of Oda Main Characters.
Next : Evil people
Evil people is first evil system : a monarchic, feodal system where the Ruling class steal people using force : Marines.
Marines are not evil, they are pawns in the system.
Real evil people of the story : the Bad Monarch, Obviously Celestial Dragons come first but we have Arlong, Enel, Crocodile and Dofflamingo being bad Monarch in their way too.
So we have good guys, bad guys.
Next : the concurents the Rivals : Moriah, Foxy, Mihawk, ( Kidd ), ( Blackbeard ). Blackbeard is not bad because he rule over a nation, he is bad because he fight good people.
And finally the Goal : people freeing themselves by attacking their oppressor even if they are weak.
We have now the Revolutionaries, idealized by Oda