It seems from what I read on chapter 718 that Doflamingo is almost for sure part of the Tenryuubito, formerly the Twenty Kings. And the Twenty Kings conquered the Ancient Kingdom 900 years ago. Does that mean the Dwarves were part of the AK? Will there be a flashback to the Void century in this arc? (I really hope so) . But when Doffy was a kid he was really dirty and such, and the Nobles are all sterile. Maybe he was an orphan adopted by the Donquixote family (one of the WN families) or he just ran away from home, like Sabo.
I dunno, but I see some epic stuff heading our way in this arc.