Possibly, But I doubt Zygarde would be the legendary. Serebii recently leaked a photo and quickly deleted it of Goomy being offered for three unknown Pokémon . Diancie, Hoopa, and Volcanion. Maybe, Game Freak(As usual) is leading us on with Zygarde. Serebii accidently leaked these so called legendaries. Maybe these legendaries will be the mascot Pokémon for a future games. Unfortunately, no pictures of these Pokémon have been leaked. But if you search Goomy in the GTS, you can still find the trades wanting the mystery Pokémon. Also I doubt Z will be the title of the next game. Probably something like X2 and Y2, or something entirely different.
Sorry for the large image.