Pokémon Sun and Moon! I’m the first to put it in your question box! Now what on earth can this be? Why are we getting gen 7 before Vishnu/Zygarde’s game?
a) No you’re not, Billy; shut up.
b) Not a clue. If I had to guess, though, I would speculate that generation VII will be more closely tied to the story of generation VI than has been the case in previous games. It seems like the logical extension of what they did with Black and White 2, at any rate. This is not to say that they’re sequels in the sense that Black and White 2 were, but perhaps we might expect further involvement of the same characters, legendary Pokémon and plot elements that were prominent in generation VI. Of course, this is just a guess at something that we objectively do not have sufficient information to figure out with any confidence at this point.
Alternatively, it’s possible they’ve pulled a GS Ball and decided to just forget about Zygarde and hope no one notices.