A major theme in One Piece is dreams. It's the motivating factor of all the Straw Hat Pirates, bar none. Many minor characters have an arc regarding dreams they have or used to have. Nearly all the villians have at least implied that their dreams have been broken in some way. It could be said that, along with ThePowerOfFriendship, One Piece runs on dreams. Let's go more in-depth, shall we?
The Straw Hat Pirates all have specific dreams. Luffy wants to be King of the Pirates, Zoro wants to be the World's greatest swordsman, and Nami wants to map the world. Usopp wants to become a great warrior of the sea, Sanji wishes to find the "All-Blue", and Chopper wishes to become the world's greatest doctor, capable of curing all diseases. Finally, there's Robin, who wants to discover the Lost Hundred Years, Franky, who wants to build the world's greatest ship, and Brook, who just wants to see that old sea whale he met so long ago.
There are a few things about this worth noticing:
1). Some crew members have already achieved their dreams. The Thousand Sunny is by far one of the toughest ships out of what we've seen. It's possible Franky has already built the greatest ship out there. Usopp is already a great warrior and a huge advantage to the team. Sanji [[spoiler: may have found the "All-Blue", although we may need more information]]. All of these dreams may be difficult to reach, but they are attainable.
2). The crew's dreams have been influenced by outside sources. Zoro was influenced by Kuina, Usopp by his father, Sanji by Zeff, Robin by her mother, Franky by Tom, and Brook by... [[CaptainObvious Laboon]]. Others have been influenced by multiple sources. Chopper had been influenced by Dr. Hiriluk and Dr. Kureha. Luffy had been [[spoiler: pressured by his grandfather to become a marine]], before opting to become a pirate after the influence of Shanks, [[spoiler: Ace, and Sabo]]. Nami's passion for navigation was a natural one, but she lost all joy for it after being forced into working for Arlong. Thanks to Luffy's influence, however, she regained her love for it, along with a newfound freedom. In fact, going back to Robin's mother, ''she'' searched for the True History out of respect for her husband's dream. The crew is living the dream that many of their friends could never see.
Now, what of the antagonists? Let's dive into their stories. It has been frequently shown that the antagonists either dismiss "silly" dreams, are living dreams that destroy the dreams of others, or have lost their dreams entirely. Crocodile is a good example of the latter example. It has been implied that Mr. 0 had once wished to become the King of the Pirates, before abandoning it entirely. He instead chose to sadistically crush the dreams of others, turning himself into an EvilCounterpart of Luffy. This also places him in the category of dismissing dreams, further separating him from Luffy. Now for the "bad" dreams: Enel dreamed of being God and became one. But getting bored with his initial dream, he then chose to destroy Skypeia and reach his promised land. Why this was a terrible idea bears no explanation. Warpol sought to become a tyrannical ruler who only thought of himself. Kuro wished to escape the pirate life, which while in itself not bad, used others a sacrificial pawns in his plan. Blackbeard is Blackbeard. None of his goals are good. Finally, we reach those who dismiss or destroy dreams. Bellamy, a believer in a era without dreams, actively mocks those who hope. He is thoroughly destroyed in a battle with one hit. A more effective "dismisser" would be the World Government. Disgusted by the dreams Gold Roger spawned from his speech, they fully attempt to crush the dreams of those pirates, along with the dreams of all of our heroes.
Dreams in ''One Piece'' serve a motivations and obstacles for many characters. Some have already clearly given up. Some never will. But the importance they play in the series is undeniable.