It seems to me that Zygarde's name is based on En garde, French for "on guard"; spoken at outset to warn the participants of a fencing match or duel to take a defensive position. Which could possibly suggest that Zygarde is getting possible new offensive forme(s)/Mega Evolutions. If we take a look at these two new moves it supposedly gets, both are physical but I like to think that Thousand Arrows will be the physical move and Thousand Waves will be the special move since sound wave based moves are usually special. Also Zygarde's stats are more defense oriented and not to mention incredibly underwhelming for a legendary Pokemon. This makes me think that it will be receiving two Mega Evolutions instead of Forme changes since Mega Evolutions significantly boost stats way more than forme changes. Now one of the Mega Evolutions will significantly boost it's attack and speed while the other will boost it's special attack and speed. This just gives off that B2/W2 sequel vibe agin but idk.
However these are just my opinions and at the end of the day just pure speculation.