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.