Blackbeard being Luffy's Whitebeard has been obvious since Jaya Arc. People are missing the symbolism for irrelevant features like being "frenemies." That's not what made them special to their generation. They're special in their generation because they're its top two pirates. Whitebeard was Roger's greatest rival because he was his strongest one and is close to him in strength. No other pirate besides Blackbeard will have that dynamic with Luffy. The fact Imu deemed Luffy and Blackbeard as the greatest rising threats is more proof that they will be the top two pirates of their generation just as Roger and Whitebeard were in theirs.
It's not rocket science. Teach's epithet is the polar opposite to Newgate's. Yeah, he has similarities to Xebec, but Xebec is to him what Shanks is to Luffy. They're their role models. What's more, Luffy and Teach's connections with their role models and old gen parallels mirror each other. Shanks (Luffy's role model) was an apprentice on Roger's ship (Luffy's old gen parallel). Luffy wanted to sail with Shanks as his apprentice (but was turned down) and ate the Devil Fruit he stole. In reverse, Whitebeard (Blackbeard's old gen parallel) served under Xebec (Teach's role model), took kid Teach under his wing, ands stole a Devil Fruit that Teach would eat.