I don't say that Zygarde will not be featured in the next movie, it's just that its role will be reduced due to focus on Volcanion (or they may focus on both, I don't know). And I doubt they will skip showing Volcanion in it, because it's his turn to be revealed this February, and I don't think GF would skip a year to reveal the last Gen VI event Pokémon because that year's movie may focus on the Pokémon Z legend. And hey, Volcanion's event in Z may be bigger than what we got with him in X/Y and OR/AS, so it would certainly work as promotion for Z.