Call me crazy but, I was watching the intro the anime and I noticed the part where they show Gol D. Roger, his nose is kind of crooked and messed up, maybe like it's been broken or something like that. And it got me thinking that Blackbeard's nose isn't exactly straight either, now in the first chapter of the manga roger's nose doesn't look quite like that so this idea may not exactly be canon but I'm starting to think Blackbeard may be related to Roger pretty closely, as close as he could be his son possibly.
I know blackbeard's pretty old, and the name doesn't match but my reasons for thinking this are first off the nose. Second off the D had to come from somewhere, and so did his dream of being pirate king which I don't think Teach is a character motivated solely by money or anything like that, maybe he has daddy issues. Most importantly, he's got a fledging mustachio, that doesn't compare to the handlebar monster of a stache that roger had, but hey hat's pretty hard to compete with.
I don't know what are your thoughts.