We all know that the 3 ancient weapons are Pluton, Poseidon and Uranus. These are supposed to be based on the Roman gods of the underworld, sea and sky respectively.
We know Blackbeard has 2 devil fruit powers, the darkness fruit (Yami Yami no Mi) and the tremor fruit (Gura Gura no Mi). Also, Blackbeard’s jolly roger has 3 skulls. He also seems to be knowledgeable about devil fruits and the history of the world. It is widely believed that he will try to acquire a third devil fruit for himself, if he hasn’t acquired one already.
My theory is that Blackbeard is actually selecting devil fruits not just for their strength, but also because they mirror the ancient weapons.
The Yami Yami no Mi is a power of darkness, same as Pluton. The Gura Gura no Mi has the power to create quakes. And on a planet where there’s a lot more sea than land, it makes sense to associate this power with seaquakes rather than earthquakes. That would make the Gura Gura no Mi similar to the concept of Poseidon.
If this theory is true, that would mean that the third fruit mirrors Uranus. We can speculate which fruit it might be, but I personally believe that it’s a fruit we haven’t seen before.
As for why Blackbeard is selecting his fruits in this way, perhaps he thinks it makes him more eligible to get the One Piece and become the king of the world.
Theory by Indigo_magenta.
 and the tremor fruit (Gura Gura no Mi). He also seems to be knowledgeable about devil fruits and the history of the world.){kind=link}