Luffy and Naruto

It's becoming madami and madami obvious that Monkey D Luffy is the future Pirate King, in the new opening song: "We Go" It shows Gol D Roger, the former Pirate King wearing Luffy's straw hat, then Shanks, who is one of the Yonko (not sure about spelling) and then Luffy.
Also his name; D, I keep hearing about the Will of D, but I'm still not sure what it is, but i think it has to do with becoming extremely powerful and having the ability to do the impossible and reach their dreams, I dunno, I'm just guessing.
Earlier on in the anime, when they were still in Logue Town, the bartender who once served...