@Daz:
or that we need to take the fight to the Wano mainland,
It ain't no baseless perception, when you consider the following;
•Onigashima is too small for the several very destructive fighters in it, just like in Marineford, everyone would be fighting too close to each other, constantly interrumping one another.
•Tama, Hitetsu, Hiyori, O-tsuru, Onimaru, and maybe Holdem and Caribou will be involved once again for sure, and they all are in the mainland(unless Caribou had already escaped)
•The citizens of Wano, or at least The Flower Capital, not witnessing the fall of its dictator wouldn't be like One Piece at all.
•New Onigashima Project, pretty much hinted a "Party's over guys" scenario in Flower Capital.
So, while not all of them would be required to move, half, or at least Calamities and Kaido would be about enough, it also goes perfectly along with Greg's ideas about Momo confronting his fear of high heights by giving Luffy a ride and a possible final round in top of Mt. Fuji.
And all of that goes without even considering that Kaido will do his own "BirdCage" and it doesn't make any sense doing it in Onigashima where most residents are his own army.