Since we're all waiting for next week and a new chapter, I thought I'd post a theory I've had for some time…
I believe that Garp is Gol D. Roger's brother, perhaps in the same way Ace and Luffy are brothers. Both are Ds, and ever since Aokiji fought with Luffy I thought the grandfather he was referring to was Roger.
Garp was always cornering Roger, but never captured him. Perhaps Garp went easy on Roger because he was his brother. It was never confirmed that Garp was the one that captured Roger.
Anyway, this is just speculation, 'cause we don't know how people get the name of "D". But wouldn't it be cool if the future pirate king was related to the old pirate king?