I have seen theories about Blackbeard taking the Cerberus, Marco’s Phoenix devil fruit or Kaido’s Dragon devil fruit. But I have seen very little about Sengoku’s Buddha devil fruit.
If the theory is about Blackbeard trying to acquire the strongest devil fruit from each category, then it’s possible Sengoku could be a target for several reasons.
First, the Marines are going to take a lot of punishment and suffer many casualties trying to take down the Seven Warlords.
This will probably push Sengoku into the fray to help his fellow Marines. This could present a moment where Sengoku becomes vulnerable or exposed by the Blackbeard Pirates when they become aware of his activities and track him down. This time the Blackbear Pirates are stronger and armed with a fleet and devil fruits that could be too much for one man.
Sengoku’s Hito Hito no Mi, Model: Daibutsu abilities have never been properly explained which likely means Oda is holding back information on how powerful it is. It’s fricking Buddha, it has to be powerful enough to help Sengoku rise in ranks to Fleet Admiral.
In addition, the Hito Hito no Mi would build up Blackbeard’s endurance and boost his defense which is presumably still an issue, allow him to properly use both the Gura Gura and Yami Yami no Mi. The size and strength of becoming a Buddha would allow Teach to fight against his fellow Yonko like Big Mom and Kaido if Luffy were to fail in Wano. Attaining the three “strongest” devil fruits from each category, Blackbeard would presumably cover all of his weaknesses and make him the greatest threat in the world.
Lastly, it would create another parallel to his rival Luffy. Luffy has been slowly been built up with Buddhist themes being hinted at like the mention of the Guardian Diety.
Oda loves drawing parallels between Luffy and Blackbeard. What better way then Blackbeard taking the Buddha devil fruit? Blackbeard attaining the Hito Hito no Mi, Model: Daibutsu would symbolize the corruption of Buddha and his teachings. The remaining Marines will be devastated to hear about the loss of their own and easily side with Luffy in a conflict against Blackbeard. It’s also a shortcut to power that has become a feature of Blackbeard’s modus operandi So I can see two towering figures representing the Dawn and Dusk fighting over the fate of the World and One Piece.
*Theory by AgentTao