So, I was actually wondering about what the One Piece could potentially be and it's quite difficult to speculate on what it might be. We do know that when it is found, it's going to trigger the biggest war in the history of the One Piece universe, which makes me actually wonder about what will happen within this war. Actually, I have an idea that the One Piece is the culmination of the hopes and dreams of countless generations. The World Government has always worked to restrict those kinds of dreams because there's an inherent fear in them that any one person or any one group may become too powerful for them to exert their influence over, which is why the Pirate King is someone who has the most freedom on the sea, since you can't exert your own influence on them.
I'm speculating that the One Piece is the sort of trigger that will allow everyone to achieve their hopes and dreams. The One Piece is a piece of technology from the Ancient Kingdom which allows one to manifest their dreams into a physical reality. For example, I can imagine that if Zoro wants to become the greatest swordsman in the world, that technology will not make him become that but will be the catalyst for him to finally face against Mihawk and claim the title. In Sanji's case, it will give him the means to truly find the All Blue, since there seem to be no leads on that so far in the series.It doesn't even have to be technology that does this, it can even be Ancient knowledge about the structure of the world. This kingdom was pretty powerful and so, it's not a stretch to say that it had a huge amount of knowledge of the world.
But here's where there is a lot of conflict. In this war to get the One Piece, especially for the Straw Hats, it's going to be a fight between each individual straw hat's dreams and Luffy's dream. It's been made apparent in the manga that Luffy's crewmates care about making him the Pirate King alot more than they seemingly do about their own dreams BUT it's a very different story when you have your own dream so close, just within your reach, and Luffy's dream. It's a more difficult choice to make. For example, I can certainly imagine Zoro having an opportunity to cut down Mihawk. A clear and easy opportunity for him to do it. Doing so will make him the world's greatest swordsman. But at the same time, Luffy is being overwhelmed by other pirates. Or his crewmates themselves are being overwhelmed and are about to die. In this case, Zoro would have to choose between having a chance to fulfill his dream or saving his crewmates.
We've seen Sabo return to the manga within the Dressrosa Arc and it certainly was a happy time for all fans. We found out that he's in the Revolutionary Army and the second in command, no less. At some point in the future, it's clear that the World Government will be taken down by the Revolutionary Army, at which point I imagine that Dragon himself would take over. Remember, we have no clue of what Dragon's motives are in dethroning the World Government. It may absolutely very well be the case that he is the final villain in the series, someone who would naturally force Luffy to obey and rule at his side, along with Sabo, with both of them being under him.
Luffy, who really won't like that will then go against Dragon. In that instance, I can clearly see Sabo having to make a choice between helping his younger brother or helping the man who saved him. The enemies are so clear currently in the series, for everyone. It's easy for Sabo to beat up someone who is his own enemy and someone who is also the enemy of Luffy, as he did with Burgess. Can you imagine what a nightmare it will be for Sabo to make that choice and that's when his loyalty to his brother will be called into question. Ace's will was to have Luffy live his life. Sabo will be torn between fulfilling that will to his late brother and his loyalty to Dragon.
I think towards the end of the series, it really will be an emotional meltdown for every character in the series. All of the relevant characters will have to make tough choices about what they want in their lives, choices which Roger's crewmates never had to make since they were hasty about it. Tell me what you think about this theory, whether you think it's plausible or not. I think it's plausible and quite befitting, One Piece has always been deeper and darker than people like to think of it.