Final guess before Leakgate2016: No split evolutions (hurts my soul to say), but a select, limited number get some sort of Z-form, starters most prominent. The twist: Starters can 'sync' with Solgaleo or Lunaala, combining to create a powerful Mega-esque ultra mon. These will be version exclusive, so two different forms depending on drawing from Sun or Moons power. It really kind of touches all bases with credible leaks; legends having vague prevo, split forms, bit truly only one stage 3 species for each. Rockruff draws power of either or to determine his evo, so there's the secret, all four interact with the rays of the Sun or moon to perfect their form, albeit in slightly different ways. Lore wise, it fits, and slides in quite nicely with Azoth speculation. RIng would need Zygarde and Marshmallows interaction, with the physical form constructed of the Earth, but the energy of Aether also necessary to act as conduit for solar and lunar rays to travel to Earth. A and Z (yes haha), from the Alpha to the Omega, and everlasting repetition. Would set A and Z up as Mythical counterparts, then Sun, Moon, and Mystery legend as trio. Azoth lore wise, could see that relating to the Dragon lurks g in darkness waiting to overtake the Sun King (bottom left corner of Azoth chart) or Tiki? I'm sure it's eclipse related, most think Marshmallow. It's relation to Z forms and the connections/counterbalance to Zygarde make me think that's not it, either. Regardless, I'm sure whichever is eclipse legend will have a handful that get their form through it. Probably Lycanroc honestly. Just thought of that, and way obvious. Werewolves, moon blocking sky, full moon transformation yada yada.

Hmm. Honestly, this is probably the most logical speculation I've ever made. Not a huge milestone, I tend to think more on the radical side of theories. But reading it over, I'm really now expecting this.