This is going to be unexpected.
Lots of pirates would have bastard sons back in the day, and leave them. So I am thinking, what about Gol. D Roger had a son before Ace?
The very man himself, Marshall D. Teach or also as known as: Blackbeard.
It’d be perfect since Roger would’ve been thirty-seven when he would have Teach. Source: http://onepiece.wikia.com/wiki/World_Timeline. I had it all checked out before writing up this theory.
I’m not going to dwell in who was the wife, because that’s useless to us. I want to explain why first. So, remember what happened in the Marineford arc? Whitebeard turned to Blackbeard and said: “You’re not the one Roger’s waiting for”, why did he say that to only Teach? He wasn’t the only pirate on the island, there was Buggy, Moriah, Crocodile or Luffy. It was a direct message to Blackbeard after he given it some thought too. He even specified that Teach was not one of those who properly carried the will of Gol. D Roger.
It was to tell Blackbeard “Just because you’re a legacy, do not feel as if you’re definitely the one.”
(I personally think it was a good personification when Whitebeard had said that their bloodline had been “extinguished” relating to Ace’s Mera Mera no mi)
In these few panels, Whitebeard claims that Blackbeard is not someone who carried the “Will of D” and disowns him from being one just like he disowned him from being his son.
You probably think that Roger and Teach look nothing a like, but when I look over the images Teach just looks like a fat roger with missing teeth and curly hair.
Relationship with Ace?
I think they both have a Ying-Yang relationship. Ace represents light, and Teach represents darkness. To completely diverse people, who are related by blood. It’s complicated to explain the Yin Yang between them. Though I think Oda showed this metaphorically between their battle:
*Theory by Luck Gandor