hmmmm, revised theory: Luffy's mother was the younger sister or daughter of Gol D. Roger. She fell in love, married, did all that good stuff with Dragon. Gol had everything in the world and had lived a long, good, life; the only thing left to obtain/explore was death. Roger and Garp most likely met up and made a deal: Roger would turn himself in; Garp would get the credit for his capture. Luffy's mother was captured by the Navy about 5 or 6 years after Gol's death (long enough to have both Ace and Luffy) for association with the Pirate King. Dragon tried to save her, she was killed during the scuffle. Dragon escaped and/or killed the Naval officers, asked Garp to raise Ace and Luffy, and set out to destroy the WG, having a vendetta, a cause, and a reason to topple the WG.