Dont know if will be a major defeat for the Alliance, but i think at least a final battle between Kaido and Luffy will happens in Wano mainland.
But what the OP say makes sense, especially about Tama, she has a connection with Ace and a fruit that is very usefull against Kaido crew, would be strange If Oda just forgot this and dont use her for something.
There's the point about Nidai and Zoro to, but could be that he just broke the Sandai in the fights and get Nidai by the end of the Arc.
The Poneglyph in the basement was the thing that i think is most mysterious by now, i think Oda is setting up something related to it, the ability of making them that the Kozukis have and Momo.
We have Yamato to, that we know nothing but seems that he's gonna be very important in the what gonna happen in the raid. O personaly think he will be somehow the key to Kaido defeat.
And there is the plot of open Wano borders, i think in one of the magazines have been some infos about Wano and the Ancient Giants, could be that Wano Layout was made by them to be isolated and by the end of the arc the country will be in the sea level because some short of destruction.