I'm sure everyone has their own opinion as to what the end of One piece will be. This is just one more.
To start off, what is the one piece? My look on it is that it's an island that only one man in the current world has ever been to, Gold D. Roger. The island contains the technology of an ancient civilization that has ceased to be, somehow due to the world government.
I find that to be obvious, whereas the next part is speculation. I also think the ancient civilization is a possible future of this world, where all the world's efforts are used to make weapons. The islands of the world came be after ice caps melted and now those islands are all that is left.
Dragon is the son of Garp and father to Luffy. Dragon is also Gold D. Roger. He is head of the Resistance simply because he knows the truth of how the government has hidden the knowledge contained on the island one piece. Which is why Garp, a marine, deems Dragon to be dangerous.
Finally, the reason that Shanks bet his arm to the "new era" is that he was once a member on Roger's ship. He must have known Roger and where he came from, East Blue. He found Luffy, a boy with a dream to be pirate king and willing to give his life for it. I'm sure Shanks saw in Luffy the traits that fit Roger.
The second paragraph most likely is just looking too far in to what the world could be.
Other thoughts anyone?