Possibly NSFW-ish…
!
Oh Sanji…Sanji, Sanji...
I'd still hit it.
I think that Luffy and Ace are going to turn out to be blood related. And I don't just mean like from those theories of all the D's being decendants of the same people and being distantly related that way. Of course there has been the idea that maybe Dragon and Roger were siblings. But this doesn't make any sense not only due to their names but also in the interaction been Garp and Roger shown in that flashback. So really, we should be looking at the two unknowns in this equation: Luffy's mother and Dragon's mother. It is not out of the realm of possibility that Roger had some siblings. Perhaps a sister? Now, depending on the ages of Roger and Garp, if they were close in age I'd say that it's possible that Garp + Roger's sister = Dragon. This would make Roger Dragon's uncle. If Garp is a generation older than Roger, then maybe Dragon + Roger's sister = Luffy. This would make Roger Luffy's uncle and Ace would be his cousin. That would be pretty insane. So, how would this make sense story wise? Well, it would possibly put a much bigger target on Luffy than Ace if he had the blood of both Dragon and the Gol D. family. Also, if Roger's relative was Luffy's grandma then it could also be Dadan. Then it would make sense that Luffy didn't live there originally and it also would make sense for a relative of Ace's to raise him. Also, maybe Garp called Dadan a good friend because they were friends originally and then lovers and then broke up after having Dragon because it wasn't working out and then decided to remain just friends. Also calling it now, Dadan's real name will end up being Gol D. Adan.