I fully believe every single move from the moment Blackbeard killed Thatch for the Yami Yami no Mi he has had a plan. I think this plan also required the Gura Gura no Mi and that’s why he stayed with Whitebeard for so long.
My current prediction is when Van Augur warps Caribou to Blackbeard. Caribou will tell Blackbeard about Pluton and Blackbeard will tell him that he already knew where Pluton was. Once Blackbeard knows the whereabouts then he might get a hold on one of the Ancient Weapons. That will indeed be a big twist on the story of One Piece.
The reason I have for this prediction is that I think the Yamato cover story will end with her seeing Blackbeards ships on the Horizon.
Currently in this conflict, we have the Strawhats which “have” Poseidon because Shirahoshi is their ally. The World government has Uranus, so it would make sense for Blackbeard to also get an ancient weapon as well. He’s going to use the Gura Gura no Mi to break down Wano’s walls and take Pluton for himself.
Also, there are all these parallelisms between Blackbeards crew and the Strawhats, both with crew number and function, except that Luffy has the Thousand Sunny while Blackbeard has not yet been shown to have a truly advanced ship.
Meanwhile, Pluton is a highly advanced warship capable of mass destruction, labeled the World’s Worst Battleship. It was constructed long ago in the city of Water 7 in the Grand Line. The battleship is said to be capable of destroying whole islands in a single shot.
Another reason why Blackbeard would steal Pluton and not Poseidon at this time is because his crew are all devil fruit users, so it would be more difficult for them to go down to fish-man island to fight and steal Poseidon . Also, it’s unlikely they can control Poseidon, since Poseidon is Shirahoshi, she would follow Luffy’s orders, not Blackbeard’s.