Decided I wanna play along with @theroadyouchoose‘s #rewrite ideas tag.
Volcanion: This could easily have been a Clemont-centric movie. He’s already floored by all the tech in it as it is. He could’ve been the one who gets strapped to Volcanion, and his tech skills could’ve figured in a bigger way - say, if the titular mechanical marvel needed repairs. Ash could’ve played a supporting role to Clemont, like he did to Max in Jirachi.
Diancie: If they’d left out Yveltal and gone for a much lighter tone and story - say, a Roman Holiday-type plot with Diancie just being a fish out of water experiencing life away from her kingdom instead of a quest to save the day because Mystical Crystal Thing - then Serena and Bonnie could’ve had a shared lead role caring for Diancie. Personally, I probably wouldn’t have enjoyed that at all, but it would’ve been something different.
Hoopa: I have no ideas for how to rewrite this one. I just wanna ask Sky for an overview on how it went down in the TRYC-verse :D
Giratina: For all my complaining about how Brock was neglected in these movies, I’m not sure I can imagine one with him front-and-center. I’ve said before that his character, by design, seems best suited to a supporting role. But there are ways to do that well. Imagine if, in Giratina, Breeder and Caretaker Brock was the one who tended to Shaymin, got her to the Pokemon Center, fed her, and looked after her. In this case, Dawn would just think of Shaymin as a cute Pokemon and butt heads with Ash over how to treat her/listen to her, and Brock could’ve been the mediator. And, when everyone starts to get sad about splitting up from Shaymin, Mature Brock could’ve put things in perspective.
And then there are the two I already talked about rewrite ideas for *COUGHshamelessplugCOUGH*