The Will of D, what does the D letter stands for? Well, let see some of the character with D letter,
Monkey D. Luffy, Jaguar D. Saul, Monkey D. Garp, Monkey D. Dragon, Portgas D. Ace (Gol D. Ace), Gol D. Roger, Portgas D. Rouge, Marshall D. Teach. They are the character named with D letter. Let see, one of the name is Marshall D. Teach, which is inspired from Edward Newgate aka Blackbeard. I found some interesting information in Wikipedia.
One early source claims that Blackbeard’s surname was Drummond, but the lack of any supporting documentation makes this unlikely. Pirates habitually used fictitious surnames while engaged in the business of piracy, so as not to tarnish the family name, and this makes it unlikely that Blackbeard’s real name will ever be known.(Wikipedia)
It was said that D letter stands for Drummond. Let us see another issue from Wikipedia. Clan traditions credit the founder of the clan as Maurice of Hungary,a Hungarian prince descended from Árpád, who is said to have accompanied Edgar Ætheling,
heir to the English throne, and his sister Saint Margaret of Scotland, when they sailed there in 1066 to escape the Norman conquest of England. This disregards accepted history that Edgar and Margaret were brought to England in 1057 by their father, Edward the Exile: Edward died immediately (some say he may have been murdered), and his children lived at the Court of England’s King Edward ‘the Confessor’ with their mother Agatha.
It was said Edward died immediately and some say he murdered (sounds like Roger), and the children lived at the Court of England with Agatha, it sounds like Ace whom lived with Ace for a while and more sounds like Luffy that live in Goa Kingdom. Since we’ve lost the history of Will of D., let’s go back to True History in One Piece, that used to be named as Poneglyph. In Chapter 203, Robin read out the text in Poneglyph “Goxilla conquered and built this kingdom in the year 239. In 269, The Dima Family is the strongest. During the year 306, Arimar completed The Fuda Holy Temple. In 325, Massamdi, The Ordian Hero…”. Hey, we found something, Dima Family, sounds familiar like Clan of Drummond. And Robin also found something at Upperyard’s forest, Robin Said “XX Year 402…That’s 1100 years ago…Time of destruction is…800 years ago…that falls into the period when there was no record of history anywhere in the world, the Blank Decade..” It means 800 years ago, something had happened before, destruction.
We know what Rouge have done to Ace, keeping Ace inside. For what? It seems Rouge do not want the world know that a new baby of Dima Family has born. Why it should be as secret? Because Dima Family connected with Ancient Kingdom by the hidden text in Poneglyph. Why should Ancient Kingdom write in code, because Ancient Kingdom fears for something.
World Government, 800 years ago created by a group of 20 Kings of Countries. The descendants is Tenryuubito. WG is created because of some kind of fear toward Ancient Kingdom. Let us see CP9. Cipher Pol 9. The crimes in the dark. Even tough it’s not time, to prevent any complication in future they usually take precautions. Behind-the-scenes secret service directly under World Government. Trained their bodies to great extreme since they were young. CP9 is made to be secret so there won’t be a ruckus in the world questioning about its existence.
Now we can conclude The Will of D. is a clan/family of lost kingdom, Ancient Kingdom, that is stands for Dima Family/Clan. Gol D. Roger fear nothing for his own execution because he already found the true history in Poneglyph, One Piece. For the true meaning of One Piece and the real history of Ancient Kingdom and World Government, I will let you imagine, read a very astonishing theory from one of mateys, or just let Oda reveal it one day. It’s better for you not to know the theory.