During Marineford, Blackbeard encoutered his “Father”, Whitebeard and betrayed him. After having killed Whitebeard, Blackbeard stole his fruit and harnessed its powers of granting mighty earthquakes. Indeed, this is the Quake-Quake fruit and Blackbeard had this power after the death of Ace, who publicly called himself the son of Whitebeard.
Ace may have represented “Christ”.
– He has 3 letters in his name, representing the trinity;
– He calls himself the son of Whitebeard, whom his subordinates call “Father”;
– Some of his fire attacks like “Flame Commandment” and “Cross Fire” are related to the Christian religion, and that of Christ.
So Blackbeard betrayed his father (like Satan in Dante’s Divine Comedy) and had the Quake-Quake fruit, after the death of “Christ” (Ace), similar to the ground in Inferno being shook after the death of Christ in the Divine Comedy.
Now, having Blackbeard resembling Satan in Dante’s Divine Comedy, and him eating the 2 strongest devil fruits, it is clear that Blackbeard will one day, eat the 3rd devil fruit: the strongest Zoan. Recall, that the 3 heads of Satan may have represented the 3 different kinds of Devil fruits
Now, based on Dante’s Divine Comedy, I believe that Blackbeard’s final form, due to the last fruit he’ll eat, will be this:
The final devil fruit Teach will take will be the “Hito Hito no Mi, Model: Devil”.
Here are a few reasons why I came up with this conclusion:
1) We see this AMAZING foreshadowing in Fishman Island, by none other than Usopp: Master of Foretellings.
“They don’t need the Hito Hito fruit, do they?”
THIS line itself just SCREAMS foreshadowing of what the Blackbeard pirates are really aiming for. Rumor had it they were hunting devil fruit users, but I believe they were hunting for this specific fruit for Blackbeard – the Devil Devil no Mi.
2) We’ve already seen Sengoku’s devil fruit in Marineford. The Hito Hito no Mi, Model: Buddha. If a human-divine devil fruit like that existed in the One Piece world, then it is confirmed that something like the Hito Hito no Mi, Model: Devil will surely exist.
Like Satan in Dante’s Divine Comedy, Blackbeard will acquire black wings, with sharp teeth and claws, and a tough hide that could probably give him protection from the weakness he took after eating the darkness logia fruit.
*Theory by Mr. Tiddles