I was thinking about how the Straw Hats could defeat a Yonko. At the current situation it is hard. During Punk Hazard, Law told that they have only 30% of winning chance against Kaido, and if they defeat Doflamingo and destroy Smile the chances would be a little bit higher. We still don’t know if that means that now they have 40% or 50% chance of defeating him.
Looking at the current arc I can see that Big Mom will not be taken down, she is too strong so I guess that they will expressly escape from Whole Cake Island, leaving Big Mom in such rage as she was never seen before.
Straw Hats will than leave for Wano Kingdom. During the Wano arc all Straw Hats’ alliances formed at Dressrosa and Zou will be there. And I think Eustass Kid, Killer and Basil Hawkin will join the alliance to take revenge against Kaido. Maybe also Capone Bege will accompany Straw Hats because they once helped him on Whole Cake Island. And there will be the remnant Whitebeard Pirates following Marco the Phoenix.
When Shanks will learn about Luffy’s plan he will come to his aid as he has once fought Yonko Kaido and realised true nature of his power. And I think he do not won’t Luffy to die so he will be in Wano arc as well.
Now the question is where in this all does Blackbeard fit?
I’m pretty sure that Blackbeard is aware of the incoming war in Wano. Since he is able to eat more than one devil fruit (maybe because of his weird body, darkness, or both, who knows), I don’t know how many fruits he can eat, but an infinite amount would be overpowered. His flag is still unexplained, but I think that it is related to an ability to own (only) 3 devil fruits.
Blackbeard is looking for the strongest Logia, the strongest Paramecia and the strongest Zoan. He already got the first two (Yami Yami no Mi and Gura Gura no Mi), and I think he will get the third one soon. Blackbeard knows what Kaido’s power is. If it is Kaido’s devil fruit (we are almost sure it’s a Zoan) which makes him so strong, I’m pretty sure that Blackbeard wants it. My guess is that like in Marineford, he will show up at the end of the war, when everyone is too weak to fight him. Most probably when Red Hair Shanks and Kaido are fighting long enough. He will do his magic trick and steal Kaido’s Devil Fruit. Steal Kaido’s poneglyph. Steal Luffy’s Poneglyph, why not, and get closer to Raftel. Wano would still be a victory for the Straw Hats, but Blackbeard would show to the world that he is the one the world should be scared of.
*Theory by Shanks_hakimaster