I have a theory that Sanji will awaken haoshoku no haki, which came to mind when I read the Germa Kingdom wiki page. According to it, the Kingdom of Germa conquered the entire North Blue for 66 days and established an empire there. Whoever did this, I mean Sanji's ancestor and Germa Emperor, could be a haoshoku no haki user. and this way Sanji may have inherited haoshoku no haki. And Blackbeard's crew includes a former king from north blue, Pizarro. I think Sanji will fight Pizarro so he can learn about his ancestor, awaken haoshoku no haki and create black flames. What do you think about my theory?