Filled with ever stripe of One Piece spoiler you could imagine up to and including the current manga chapter (579) and anime episode (463+).
Eiichiro Oda just recently announced that One Piece is going on a month-long hiatus after chapter 597. So as the manga moves on to the next stage I thought I would be the perfect time for me to reflect how I think One Piece will end. As I have expressed on the blog before I enjoy trying to figure out what the mangaka has planned and see how many of suspicions are correct. Oda plans out his manga enough that I feel this is viable thought exercise because he is not just making up the story as he goes along. Everything below is my speculation but I feel my theories have a good deal of contextual proof and hints backing them up as well. So let us sail to the end of the Grand Line and see what is there.
After the time skip the Strawhats will meet back in Sabaody, go to the Fishman Island, get in trouble, get a fishman or mermaid crew member, and continue to the New World. The will have 3 or 4 major arcs as they go through the New World including one big adventure the island of Giants, Elbaf. They will fight the marines, the Shichibukai, the Yonkou, Supernovas, and various as of yet unknown threats. Eventually they will get to Raftel the last island on the Grand Line. There we will learn the secrets of Void Century, the Ancient Kingdom, the Will of the D, and many more secrets.
Raftel is what is left of the capital of the Ancient Kingdom, a super technologically advanced nation that was wiped out by the organization that would go on the be the current World Government. There have been several hints throughout the series that the Ancient Kingdom had hyper advanced technology. We know they created the unbelievably powerful Pluton and Poseidon weapons and were involved with the cities on the moon. The World Government does not want anyone learning about the Ancient Kingdom for a wide variety of reasons including their hand in its destruction. Everyone who has the Will of D is connected in some way with the Ancient Kingdom and the D in their names is a remembrance of their fallen homeland.
The reason that no one other than Gold Roger has made it to Raftel is that the island’s hyper advanced security system is still active and destroys ships that come too close. Therefore the major arc before getting to Raftel will be the Straw Hats find away around or through the security system. When they finally get to Raftel any parts of the history of the Ancient Kingdom not revealed will be brought to light. I also suspect that many of the dreams of the Straw Hat crew are somehow tied to Raftel. Maybe the All Blue is actually a super advanced aquarium on Raftel or there is a device that has a map of the world for Nami.
When the World Government finds out that someone has finally breached the defenses of Raftel is when the series will start wrapping up. The World Government will want to eliminate Luffy and his crew for discovering the truth and Luffy will probably have good reason to want to take down the World Government as well. Eventually by various means it will be the Straw Hat crew and their allies vs the world. Expect epic battles and the return of pretty much anyone who was anyone on one side or another.
I except that somewhere along the way we will find the true final boss of One Piece is none other than Marshall D. Teach aka Blackbeard. I expect him to swoop in when both sides are exhausted from fighting each other. Oda has been slowly building Blackbeard up as the dark counterpoint to Luffy. We will learn that Blackbeard is a descendant of someone who helped sell out the Ancient Kingdom to Twenty Kingdoms thus fully solidifying him as a Judas type character. He will get one of the ultimate weapons and my money is on the Pluton just because it seems to fit his theme. Every member of the Strawhats last battle will probably be against their counterpart in the Blackbeard pirates.
I expect Shanks and/or Dragon to die before the end. Anyone else could die but I think Oda will stick to a few key deaths more than a Tomino style murder spree. The World Government will be overthrown and Blackbeard will be defeated. Maybe being the hero of the day Luffy will be offered a place in what ever government forms to replace the fallen world government. I assume he will wisely turn down such an offer and continue to explore the world with the rest of the crew continuing on with him. It has a nice mixture of resolution and yet a sense that the journeys are still continuing. (Plus it leaves them room to make movies and bonus adventures which I am sure Shueisha and Toei Animation would love.) The story may be over but the adventure never ends.