So basically it appears that Zygarde takes on semblence of different life-forms in different stages of power.
It starts off in the core of biology, as single-cell-creatures.
At 10% it already takes on the form of a more regular animal, in this case a dog.
At 50% it is the majestic looking serpentine design we already knew the whole time.
At 100%, it becomes a shadow overlord kaiju-mecha-perfect-cell monster! Which is supposedly stronger than Xerneas and Yveltal specifically (perhaps a hint at a conflinct between them in the climax of the XY-sequel/s as I've been speculating about before?)
Furthermore, the creatures picked for the designs seem to be based on (Norse mythology of course) Lokis children:
-Fenrir, the wolf
-Jörmungandr, the serpent
Now what is Hel you wonder?
A "humanoid" goddess of the dead.
Often described as being half human and half blank (which explains the perfect forms humanoid posture, and Zygardes half-colored-design-aspect), or as half alive and half dead!
That is perfectly captured with the blue and red hexagonal patterns on it's "wings", as they represent Xerneas (the legendary of life) and Yveltal (the legendary of death)
(interestingly, the colors in that order happen to resemble the french flag, which is especially evident on its chest)
"Hel in a reading can represent a time of simultaneous endings and beginnings, the point at which the circle is completed. She can also indicate integrity, as opposites unite to form a stronger whole."
Perfect enough so far I would say.
The beginning and the end, at the same time. Those 2 were a big theme in ORAS, the alpha and omega. As well as the character AZ in XY, and Zinnia/Aster (the first and final letters), it's really tying all the elements together it seems.
There is also an interesting detail visible in the Anime-Poster. The spectrum-line on Perfect-Zygardes chest can actually open up to reveal 5 hexagons!
Considering that we have had 4 games so far in this generation, and both times it was shades of red and blue.. maybe the hexagons represent the four legendaries that bring imbalance to the ecosystem if they awaken?
With white in the center representing Zygarde itself? (as white hexagons are generally a feature of its design). But inluding Groudon and Kyogre, while excluding Rayquaza (although that did also happen to it in ORAS, being shove aside into a post-game quest) would be strange.
|A Zygarde-Cell with an inactive core|
But there is something else these 5 colored hexagons could represent...
Let's get back to one of Zygardes forms. The Cells. There is 2 kinds of those, ones that look like they're in a weak state, and ones that look lively.
|A Zygarde-Cell with an active core|
Could there be 5 Zygarde-"Cores"? Red, orange, white, cyan and blue.
I can certainly imagine the climax of the plot for the upcoming game to be a big battle between Xerneas and Yveltal or something of equal importance, that brings big "imbalance to the ecosystem", threatening to destroy the world. And we as the player having to go search for Zygarde.
But with the twist that we need to collect all of the Cores scattered in the region somewhere. Zygarde would then morph into a stronger and stronger creature with each Core that is gained, and finally into the Perfect Zygarde, with the 5 chest-hexagons representing the 5 Cores. All of this is pure speculation of course.
Southern Kalos is a high possibility for the game. Assuming it follows the same structure (coast-central-mountain), the Kalos pentagon symbol might change to something like this:
If that were the case, the obvious thing to do would be to have us look for one Core in each of them.
Can't wait for a proper Sugimori art instead of Anime-art, but I love where this is going.
I'll leave you with something I couldn't resist doing.