Just wanted to get a theory out of my head and hear your thoughts about it. Feel free to flame or nominate for worst thread of the year if you like.
Anyways, theory/speculation being this: Franky's father = Kaidou.
1. The cyborg henchman of Kaidou's we saw being attacked by X-Drake in the new world. If we assume Kaidou built/modified the cyborg himself he'd have to be pretty skilled engineer, just like Franky showed a talent for even as a small child when he at around 5 (not sure and too lazy to check now, but small child anyway) built a cannon out of scrap. Learnt/picked up from his father/in his genes or something similar?
2. It seems unlikely to me that a pirate would leave his child behind in the middle (or further) of the first half of the grand line - and then NOT keep going and aiming for the new world. For whatever reasons he was abandoned, protection from the dangers of the new world or just a pain in the butt and bad toilet scrubber, do you think it's likely they would drop/throw him off the ship and THEN try and get out of the grand line through the calm belt or something like that?
3. If Franky's father was dead or in prison, why would Oda even give us the information that he was a pirate? When does he really give us unnecessary information like this he doesn't later use and we realize it was actually a clue to a twist he had in store for us? And along with the cyborg subordinate of Kaidou - considering how rare the cyborgs have been so far in OP - maybe there's a connection.
[B]Unlike other family members you might want to bring up as a counter, such as saying "in that case what about Luffy's mother and grandmother, Zoro's parents, Sanji's parents etc etc~" - in this case Oda has actually given us some information, even if it is in the small form of the pirate identity.