1). Zoro's Past
What exactly happened here, we know he made a promise to be the world's greatest swordsman and he lost a friend along the way, but what happened before that all we know is he turned up at a dojo? Did he lose his memory or was he apart of some sort of greater trajedy like Sanji was? In sure we will find out soon enough.
2). Blackbeard Has Two Devil Fruit Powers.
Now this is an interesting one, we know if any character was to eat more than one Devil Fruit then this will develop a sort of poison and be the cause of their death. However Blackbeard was able to get hold of two powers which is something we might figure out later on through the series.
3). D
As the series reaches it's climax with Big Mom, one thing has always played on my mind, what is the D and why are these guys special. Not only this the world government and everyone within it are scared of the power it could possess in the future.
4). Straw Hat
This one is interesting guys but will be a mystery unless my theory comes true, will Luffy ever return the Straw Hat to to Shanks? We all know Gold D Roger was it's first owner so will we ever see Luffy return it or will Shanks die?
5). Void Century
This is important guys, and the whole story kind of revolves around it. Nico Robin is the only one capable of reading the poneglyphs and she is on a mission to learn about what exactly happened during the void century. This is a mystery we need to know about and will be important in descovering the truth about the One Piece.
6). One Piece
This is the greatest mystery of them by now we probably heard several different versions or theories of what one piece is, but honestly I just think it's "Freedom" Luffy will eventually destroy Fisherman Island as the premonition revealed, and will merge all the seas together to make one world where humans and fisherman can live in peace. Notice how the All Blue will be formed after this happens? Makes the most logical sense but until it happens we will never know.
Happy steeming loved ones kindly share your thoughts!