This might be overly complicated conjecture, and there’s no substantial proof in my favor for this theory, but watching the anime again, and keeping up with the manga, I’ve come realized an interesting revelation.
So, once we’re in Jaya, we’re introduced to Blackbeard, Marshal D. Teach, who is an inspiration to Luffy in the very moments between his confrontations with Bellamy, and he states that the Dreams of Man will never die.
A noble thought. Leaving him behind, Nami comments on him being weird, but is corrected by Luffy and Zoro that it’s not He. It’s They. The speculation back then was probably that Luffy and Zoro could tell that all of his Nakama were there on the island with him.
Later on, we receive more comments about Blackbeard’s constitution, even from someone that might know first hand, his former Doctor, Marco the Phoenix.
Marco says that Teach has a very unique body, different from anyone else. It is Atypical. This fact alone is what most characters, and us, the readers, attribute to his ability to wield two Devil Fruits, and why we imagine we will eventually see him gain a third Devil Fruit in the future. So far, he has the ultimate Logia. The ultimate Paramecia. We assume he is looking for the ultimate Zoan.
The theory I propose does not actually revolve in that particular Zoan-type being his third Devil Fruit, but his fourth. That’s right. He has other powers, already, that we don’t know about. Or, at the very least, he benefits from the abilities of a certain fruit and their previous user.
Forty years ago, Blackbeard was born. That much we know. But what was happening around then? Well the Rocks Pirates crew was in full swing, had gathered up nearly every powerful pirate they could get their hands on, and were running a massive operation from the Pirate Paradise island. Rocks D. Xebec had it made. He even possibly had made time to start a small family.
So far, things line up just fine. Blackbeard is born, Rocks D. Xebec is nearly the most powerful pirate in the world, and just two years later he faces defeat at the hands of Garp and Roger.
Rocks D. Xebec more than likely dies in this moment. But just like Brook after his solo, he doesn’t die. Rocks could’ve been in possession of a fruit that halted that process, but does not send him back to his own body. You see, Rocks D. Xebec had eaten the Sere Sere no Mi(or some similar name) which would let him possess another man’s body, but only if he had already perished.
Lightning round. Rocks D. Xebec possesses his son, Marshall D. Teach. Blackbeard has two entities inside of him. This is the origin of his Atypical body, why Luffy and Zoro knew that Blackbeard wasn’t just Blackbeard. The Sere Sere no Mi respawned in the wild, or possibly was lost on God Valley, thus the two souls can technically both carry a fruit (Not sure how this helps blackbeard’s third fruit theories).
This explains Blackbeard and his often conflicting personalities. Sometimes he is very courageous and confident, and others he can seem very stupid, and cowardly. These changes are at the drop of a hat. The brave aspect is most likely Rocks shining through. This explains a lot of Blackbeard’s knowledge on Devil Fruits, and his instant expertise with the Gura Gura no mi.
*Theory by nixiera