Thanks to a hacker, we can be almost 100% sure Zygarde will get a new form(es). The hacker discovered 3 new moves in X and Y's RAM data. These are;
Some move similiar to Celebrate, something like Hold Hands.
Thousand Arrows, Ground, BP 90, Acc 100, PP 10.
Brings down all Flying and levitating Pokemon
Yhousand Waves, ground, BP 90, Acc 100, PP 10.
Traps all foes that got hit
These moves sound like Zygarde will get 2 new forms, Life Form? Destruction Form?
Life form could have Thousand Waves replacing Zygarde's Land Wrath. And become Fairy/Ground.
Destruction form learns Thousand Arrow , and is Dark/Ground