Here is what people do not understand. The fact that Luffy said that he will beat all the emperors, doesnt mean it has to be the current emperors. If Kidd , replaces Big Mom as a yonkou, Luffy and Kidd can settle the score in Elbaf.
In my opinion it will go like this.
Kidd defeats Big Mom and Luffy defeats Kaido.
Kidd steals Kaido's polneglyph and runs to Elbaf.
Luffy beats Kidd in Elbaf, while Blackbeard kills Shanks in Elbaf.
Luffy vs Blackbeard in Laugh Tale.