There is a slight chance that Sun and Moon are connected to Zygarde.If you look at complete forms four chest cores, the colors are red, blue, cyan and orange. We had X for blue and Y for red, now we've got Sun for orange and Moon for cyan.It might be that Zygarde represents both Z and Earth. So he works as the third legendary in both XYZ and Sun Moon Earth.Or maybe... maybe the other 50% of Zygarde actually comes from the third member of the new legendary trio? In any case, there is still plenty of hope