This chapter really opens the gate to a lot of possibilities for the future.
First of all, the "Will of the D" is clearly something really important to the plot now. Second, the Marines will now have their hands full trying to recapture the level 6 prisoners. Makes me wonder if Magellan is okay or not.
The most important thing about this chapter, however, is the exact nature of "One Piece".
From the chapter, "One Piece" might be a weapon or knowledge left from the forbidden history that can be used to destroy the World Government. If it's a physical device of a sort, it makes you curious as to WHY Gol D. Roger didn't use One Piece. If it's a good device/weapon, I'm cerain Gol D. Roger would have protected it and kept it a secret.
That is why this chapter makes it fairly clear that "One Piece" is a piece of information. As suggested by Silver Reyleigh, One Piece might be the forbidden history and other knowledge of the old.
Given the way the flashback is set up, "One Piece" might be "Dream". The "Will of D" could be interpreted as "Will of Dream". The idea is that the journey toward your dream helps you grow as a person and find what you are truly looking for. This would make sense as to why at the end of his life, Whitebeard decided to declare the existence of "One Piece" as reality. Whitebeard has has achieved his dream, and he is taking this opportunity use "One Piece" to inspire his crew and others in the world to seek out this treasure, their dream.