If Sabo was Blackbeard, Ace would have recognized him. Sabo is sure to have survived that blast though. A more probable twist would be that Sabo ends up as the 4th Yonkou or one of his men.
Hopefully this is the end of the flashback. There is no One Piece next week and I would have to have to wait 2 weeks for another flashback.
That or Oda will make it an offscreen death like what he did in Chopper's flashback. This flashback is coming to an end in probably a month. There's no one piece next week, regrettably.
If Sabo was Blackbeard, Ace would have recognized him. Sabo is sure to have survived that blast though. A more probable twist would be that Sabo ends up as the 4th Yonkou or one of his men.
He could also be part of Dragon's crew. Dragon was pretty much there during the fire so he could still be hanging around somewhere close.