I have to say that it is incredibly unlikely that Zygarde would get a mega evolution since, to my understanding, Game freak said that no 6th gen pokémon would get mega evolutions.
I feel that what happens with Zygarde will be similar to Giritina and White/Black Kyrum. It can get a different form change depending on what area of the world it is in or what item it's holding, like Giritina. Then like what happens in White/Black Kyrum its signiture move would just change into its new one if it had its signature move learned. Those are just my thoughts about Zygarde
I feel that what happens with Zygarde will be similar to Giritina and White/Black Kyrum. It can get a different form change depending on what area of the world it is in or what item it's holding, like Giritina. Then like what happens in White/Black Kyrum its signiture move would just change into its new one if it had its signature move learned. Those are just my thoughts about Zygarde