>>58907222We know from the teraleak how Zygarde cells were planned to work, and the rocky process in implementing them.
>XYOriginal plan was to collect Zygarde cells scattered around at the postgame-only Southern Kalos towns, and then assemble it at Terminus Cave. With South Kalos cut for the "Kalos Reserve" games they shelved the gimmick to that.
>Unused Kalos Reserve gamesZygarde was supposed to get two different "complete forms" upon completion of the story. Never saw the light of day obviously, to switch gears to SM for the anniversary.
>SMWould have revealed that Zygarde was born/hatched in Alola. It was supposed to fight Guzzlord as its natural enemy, and also battle Parallel Lysandre when the Rainbow Rocket story was planned for SM instead of USUM. The collectathon was (finally) implemented, but the more story heavy elements and lore additions were completely scrapped.
Then USUM half-assed the collectathon and ZA just had Lysandre do it for you offscreen, presumably because of the all the dev hassle from 2012-2016.