I long time ago late 2007 I made a prediction about the D bloodline. I think its time I reiterate and add some more.
Here goes…
The entire Poneglyph puzzle contains information regarding the ancient civilisation that was destroyed by its current opposition which is the Government in power today. The Poneglyphs not only reveal the name of the extinct civilisation but also mentions the location and the mighty leaders that ruled it. Those rulers all being from the infamous bloodline D.
The Government fear the D's and the history due to the possible revival of the kingdom, the atrocities the Government created, and the Ancient weapon used to wipe them from existence.
One piece itself is actually the person that will become the terrifying manifestation that reveals the history and ends the Governments reign of power, thus freeing the people/lands/seas. When Roger said he left everything in one piece, he meant he left the information and burden of the long extinct nation for the future saviour to uncover. When Luffy uncovers the puzzle of the void century, he himself will become the entire piece referred to as one piece.
The reason WhiteBeard wanted Ace to succeed was because he thought that he was the saviour Roger referred to.
Roger: My fortune is yours for the taking… but you'll have to find it first
I left everything I own in One Piece!
He could have been referring to the heir that he thought would be Ace. But wanted to make a chaotic new era that would keep the Government busy and give Ace a chance to move on unnoticed for a while, making his journey to becoming One Piece easier..
But both Roger and Whitebeard were wrong, the real heir to this position is Luffy.
Whitebeard: I sometimes see men with that D in their name… I got one on board named Teach. What is this "D" all about?
Roger: Oh, you wanna know? Alright, I'll tell ya....
Roger: It dates long, long time ago.....
Whitebeard: You're not the one....
Blackbeard: !? You're still alive!!?
Whitebeard: You're not the one Roger is waiting for... Teach, not you.
Blackbeard: Eh!?
Whitebeard: Just as there's someone who will carry on Roger's legacy... in time one who carries Ace's will will appear as well... Even if you destroy the bloodline... their flames will never be vanquished.... That's how it's been passed down the generations from long, long time ago.... And in the future... one of these days, someone will appear with the burden of the centuries on their back, and declare a battle onto the world.
Sengoku.... you and your World Government is terrified of that inevidable battle that will bring turmoil to the world!!
Although I have no interest in it... when someone finds that treasure... the world will turn on its head!! Someone will find it out, that day WILL come.
Whitebeard: THE TREASURE OF ONE PIECE EXISTS!!!
The main point drawing a conclusion about the heir/saviour being becoming One Piece seems a little far-fetched…...but I'm thinking something along these lines/similar will occur. Hopefully we'll see some more information revealed from either Blackbeard, Sengoku or Robin that will help clarify my theory, or direct it towards the true answer to Oda's puzzle.