A
Oda wants to finish the story faster than we imagined so he has Kaido and BM make an alliance , and will let them down after this arc. Shanks and BB will begin their battle and Shanks will lose but Luffy will see him either before he dies or he will save him and take on Black Beard. maybe Black beard would like to go after Luffy and his alliance while they are still recovering from the battle but Shanks is going to be in his way , and that's how the their battle will begin and they will fight for days till Luffy recovers and will rescue Shanks or avenge him.
