As far as a story goes for the Purore region idea I'm playing with...apart from the usual Pokemon League storyline (and I'm still blanking on a villain team idea), I wanted there to be a storyline based around the research of Pokemon types, the Typergy phenomena, and where all the types originated. In addition to the box legends that represent physical elements (like Grass and Rock) and energy elements (like Fire and Ice), which are ultimately the hotspots for all the types they're connected too, each of the 19 types (including the brand new Love) are connected in origin to 19 - count 'em, nineteen - different legendaries.
These 19 Legendaries act as bosses in the storyline like SV's Titan Pokemon. The Normal Legendary would be the first you'd challenge, before giving you open-world reign to fight the others in whatever order, with their levels scaling depending on how many you've faced before. These 19 Pokemon are all pure-typed of course and they act as representations of their type in their purest form...whatever that means. They can't be caught in these initial fights, but once you beat them the first time, they'll leave their 'boss arena', allowing you access to the Typergy Crystal of their respective type that they're guarding, and can be found roaming the open world Wild Area from then on for you to catch.
21 legendaries sounds like it'd be a super-cracked number for one gen, but oh well, fits the fact that I'd want the Purore games to be super expansive.