It is a compilation of theories I read in a lot of forums, not mine, but am too lazy to check whose it is. Mostly from Japanese OP Wikia. Will elaborate it a little and give my little two cents.
There's a lot of theory about One Piece's ending and what is One Piece. I do think it is related. Logically the manga will end when One Piece is found. I am sure everyone remember the ancient unnamed still Kingdom from void century. It is indicated that the kingdom is so big, yet no one appears to remember a single thing about it. It is also hinted that the current world government is an enemy of this kingdom. There must be something that the kingdom did that made the 20 nations united (thus later became the world government), opposed them and lead to the kingdom's downfall.
The theory I read is like this: The kingdom wish to unite the world into one, destroying the red line. Their logic is that with red line gone, it is easier to commerce or voyaging from all over the part of the world. The ancient weapons is prepared for such project. They will use Pluton to destroy the red line. With Red Line destroyed, the magnetic and weather line is disrupted, and it is Uranus's duty to control the situation. Then what Poseidon do? Our mermaid princess will command the sea kings to carry Noah taking all of the Ryuguu's kindom to the land above. Why? Since it's a given the kingdom could not stayed there if Red Line is destroyed. So Joy Boy's promise is to take the kingdom to their longed sun.
In short, one piece is a the ancient kingdom's project to unite the world into one, one piece of the world (very bad pun, am sorry).
The kingdom is destroyed maybe because :
1. Another kingdoms does not want the ancient kingdom to dominates the world
2. They found the plan to be too risky
Both of the reasons is reasonable. Personally I found that if the reason is true, then what the GV did back then is quite justified. But maybe they've gone too far and destroyed the kingdom altogether? This is the reason why Arabasta did not join the tenryubuto. The sand country is such a good nature one haha.
So by going even further, the mugiwara will found out about the project. But personally I can't find Luffy do the plan, even if, by another theory, by carrying the name of D, that maybe is the Kingdom's royalty line. I just can't picture him to do that. I do not get his probable motive to do that.
tidbits 1: By this theory, many assume that Uranus is Eneru's Vairy Fearth (the moon). Since he found the civilization up there, and moon is naturally weather controller (and has gravity to be used (?))
tidbits 2: With Red Line gone, All Blue is created in the place where all the sea's meet (previous reverse mountain). Sanji will found his dream. Nami would also be able to draw the world map (since it's the end of new world and another landmark is created). Another crew member's dream is fulfilled to since the journey is finished.
The theory seems adequate for most part. I do find some debatable aspect, but the thread is about what is one piece so I will stop here. I am sorry if my words are hazy. English is my 4th language.