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It would be cool to fight the UB's on their own Ultra Spaces individually, but I really hope in the story itself, we see all of them come to our dimension for a huge scale invasion
One of Sun and Moons most interesting plotlines was that Nihilego was infecting the Aether Foundation and brainwashing all the grunts, maybe this time, we can see that spread even farther, maybe all over the region and they can go nuts with the design of the infected people and pokemon, maybe it even infects some of our friends/allies who we are then forced to battle
Have all the Ultra Beasts come out at once all over the region, show the effects of what they're doing and make it feel like a really high stakes situation, and show as many characters as possible trying to deal with them (Kukui, Looker, Anabel, Trial Captians, etc.)
Zygarde awakens and combines to his full form to hold the line to repel the UB's, and all the other old legendaries arrive too as they sense the danger
Necrozma is either the savior or their leader of the crisis, and during the final hour he shreds his prison prism armor and unleashes his true light form to either repel the UB's or finish us off once and for all, transferring his armor to Nebby who either accompanies us, or is forced to battle us against it's will
After the crisis ends, we can go catch the Ultra Beasts by traveling to their respective Ultra Spaces
tl;dr; I wanna be able to play Komm Susser Tod in the background and think "Hey, this really fits!"