The Ultra-Beasts, I have a plan for
They'll basically have a mini-chapter of their own, each one having it's own little side scene
The MC won't capture them since they're supposed to be common in their home-dimensions, Maple also says that they don't belong in this world so saving them isn't part of the mission
Necrozma will be caught though as it's classed as a Legendary on the Serebii page
The page I'm using for anyone interested
I'll be using a similar idea for Paradox Pokemon too