Many people speculated that Blackbeard will invade Whole Cake Island to get the Road Poneglyph and kidnap Pudding who as member of the three-eyed race has the potential to unlock the ability to hear the Voice of all Things. Blackbeard will perhaps fight Katakuri in the process.
After that, Blackbeard will take his third and final devil fruit. As we all know, Blackbeard has the most powerful Paramecia being Gura Gura no Mi that has immense destructive power and the most powerful Logia being Yami Yami no Mi that has the power to nullify any effect of Devil Fruits, and has the power to maintain large gravity inside his logia.
So Blackbeard’s third Devil Fruit should be a Zoan type fruit. Most people agree that the most powerful Zoan type devil fruit at this moment is Kaido’s Dragon type fruit.
We all know Blackbeard is an opportunistic machiavellian character. He also is aware of the recent developments in Wano.
All Blackbeard has to do is go to Onigashima, wait until everyone is fighting and use the powers of Shiryu and Catarina Devon which are perfect to infiltration missions to get Kaido’s Poneglyph.
Then sit down a little, and after the fight is over, when everyone is weakened, take Kaido’s Zoan devil fruit like he did to Whitebeard in Marineford.
Blackbeard has shown himself to be an expert planner and a master at orchestrating manipulative schemes provided he has a sufficiently advanced period of time to prepare. I think Wano will be no different.
*Theory by Pirate_Indigo98