My dream:
Ryokugyu is gonna get in the middle of the party, before anyone notices him and reacts, he is gonna get a bunch of people with his vines, like Tama, Toko and some other people from Wano during the party. Then he is gonna start talking with the Straw Hats + Kid and Law. Talking about the events of the Reverie and to finish things off:
"Either you come with me Strawhat, or Sabo and Vivi are dead, i can also take down a few of these kids here"
Luffy doesn't have another choice but to accept. Luffy surrenders and goes with the marines.
Straw Hats have an arc where they go to Elbaf to gather the giants army (Usopp's moment) and also call on the Straw Hat fleet. They can also ask for Dragon's help from the revolutionaries, to save both Sabo and Luffy.
Then we get the first half of the war without Luffy and Sabo, but getting nice moments for all the Straw Hats and Law + Kid.
Zoro, Sanji and Law + Kid can fight the admirals.
Usopp can have a moment leading the Straw Hat fleet to complete his lie of having 8000 men following him.
Then after a hard ass fight, they release both Luffy and Sabo, who together take on Akainu.