After Roger found the true history of the world, he disbands the Roger Pirates. After this, he has a meeting with his rival, Whitebeard. He reveals to Whitebeard that his real name is Gol D. Roger. Whitebeard was curious about what the D. means and tells Roger that he has a crewmate named “Teach”. Whitebeard didn’t reveal Teach’s full name to Roger, this always bothered me. You were talking about the D. initial. Why not use his full name which contains the D. initial you are talking about?
Roger then reveals the Ancient history to Whitebeard. I believe during this reveal, Roger told Whitebeard that someone bearing the name “Marshall D.” betrayed the Ancient Kingdom, or caused massive chaos in the past.
THE ONE IRONCLAD RULE OF THE WHITEBEARD PIRATES
The rule was “Do not kill a family member”. Blackbeard violated that, but when Ace wanted to bring Blackbeard to justice, Whitebeard doesn’t let him go. Why? Blackbeard violated the one rule. Normally, I would imagine Whitebeard would be furious. But this time, he just wants to let it go?
What bad feeling? Why bad feeling? I think it’s because of Roger’s story. If Roger told Whitebeard about there being an evil Marshall D. in the past (and maybe even telling him how a Marshall D. might bring about chaos in the future), it would make sense for Whitebeard to “have a bad feeling”. The descendant of a man who made the ancient world chaotic, is now beginning to rise. Of course, he wouldn’t want his son to get involved and put himself in danger.
NOT YOU
Whitebeard proclaims that the one Roger is waiting for was not Blackbeard. He says that with confidence. The wording is important. “… but not you, Teach, you are not one of them.” It never was Teach. Teach never had a shot at carrying the burden from 800 years ago.
Whitebeard was never insistent on making Teach the Pirate King like he did with Ace. Why? He is a D too. So why Ace and not Teach? You could say it was because he was Roger’s son. But when Ace told Whitebeard that he was Roger’s son, Whitebeard never gave a shit about that.
Because Ace had a strong ambition to become Pirate King? No, Ace abandoned that ambition.
The conclusion I draw is that Whitebeard thinks that someone with the D. initial must be the one who carries Roger’s will, but it was “not Teach, at the very least” because the Marshall D. family members were destined to cause chaos and not the people who would bring about the Dawn.
It also gives some good depth to Whitebeard’s character. Even though he knew Teach was meant to cause chaos, he did not kick him out of the crew or treat him differently, because after all, until he does something evil, Teach was his son, and he loves all of his sons equally.
Theory by OkUnderstanding3669 (https://www.reddit.com/user/OkUnderstanding3669)