I hope you realize this makes no sense from a story telling perspective..
But hey, we'll see what happens soon enough..
Don't talk about making sense from a story telling perspective because your story line is as dull as it gets. It sounds like the grand war of Naruto or a random arc in Fairy Tail. No matter what through the power of friendship and inherited powers we will overcome even the strongest enemies.
I've already said this a million time on this forum but I'll give the short version one more time. This is my theory:
The Yonkou war arc started on Punk Hazard when Luffy and Law made their alliance to take down the yonkou. It kicked off as soon as they captured Ceasar Clown.
Ceasar Clown is the one that completes Kaidou's army by making smiles and completes Big Mom's army by developing a giantification method.
The first one down the line that was directly influenced by the capture of Ceasar was Doffy as the underworld broker, because if he didn't deliver Kaidou would be coming for his head. By taking him out, they put the wrath of Kaidou on themselves. Luffy already started a fued with Big Mom on Fishman island. To complete the parallel both of them have a road poneglyph which means they have to visit both islands.
So I believe that the Whole Cake Island arc is Big Mom's introduction arc. In the mean time the story is not really that much about Big Mom but 75% about the Vinsmokes. At the end of this arc the Vinsmoke story will be dealt with. Seeing how the next arc is Wano I think we're going to see a similar set-up where Zoro and co will explore the island and deal with the Samurai storyline with Kaidou in the background. Making the Wano arc the Kaidou introduction arc.
At the end of both arc the SH's have thrown a last big wrench in the plans, leaving behind 2 furious yonkou coming after them. But in the meantime almost all the Supernova's have gathered over the course of the last 3 arcs with the exception of Bonney. I think the supernova's will stir up the world once more much to annoyance of the WG because they will be the cause of a yonkou clash.
I think Big Mom will collide with Kaidou when they're after the strawhats and a war will spark between them over Ceasar Clown and the other road poneglyph.
in my opinion THAT is good story I like to see being told in Oda fashion. I don't think Luffy is ready to face a yonkou, his yonkou win will come versus Blackbeard.
Also, what I'm willing to bet if Luffy beats Big Mom in this arc ? I will change my avatar to a picture of Zoro. That would amuse a lot of people here I thinl. But I'm confident I'll have Sanji there for the rest of the year.