I found a very interesting reference to Luffy's grandfather here.
Nothing is known about Luffy's grandfather. He only appeared in a 1997 ad for One Piece in Weekly Shonen Jump. Luffy's Grandfather was also featured in Eichiro Oda's pilot version of the chapter Romance Dawn; originally, Luffy's Grandfather, a Pirate Captain in his own right, gave Luffy his strawhat when Luffy began his own pirate adventures. Luffy's Grandfather was also responsible for providing Luffy with the Gum Gum Fruit, which Luffy consumed to gain his signature stretching power. This was re-written for the actual series, however, in which both the Gum-Gum fruit and straw hat belonged to Red-Haired Shanks, Luffy's idol and role-model.
This could actually end all the theories that gold roger is Luffy's grandfather, if Oda has stuck to his earlier version.. in that case, grandpa is probably just an as of yet unknown pirate who we'll no doubt learn about more.