We now have the Sugimori art (albeit in low-quality photo form) for the cover legendaries for Pokemon X and Y, which a mod told me I can post here -- Xerneas the stag, and Yveltal the eagle. And the evidence is mounting they are based on Norse mythology, as explained in this superb illustrated article.
After reading this article, it seems clear that Xerneas is based on the stags around the trunk of the world tree, Yggdrasil, residing in Midgard, the realm of men, each with a different colored gemstone in their branch-like antlers -- red, blue, purple and gold. ALL of which are represented in the antlers of Xerneas, depicted in the trailer by a massive tree, looking skyward like the stags of legend...
Artwork for the starters and legendaries is here.
Bigger versions up to 2700wide at the link.
...to Asgard, the realm of the gods, at the top of Yggdrasil, where resides the Blind Eagle, another protector of the realm, who is in all likelihood the basis for Yveltal. The article even notes how the bright blue tint to Yveltal's eyes could denote his blindness, in keeping with the myth. The myth also holds the Blind Eagle can see into the souls of all living things across the nine realms connected to the tree.
And finally we have what lies under the roots of Yggdrasil, trapped near the realms of the dead, the serpentine dragon Niohoggr. You can bet this will be the basis for the third legendary, antagonist to both Yveltal and Xerneas, and in all likelihood, the first Dragon/Poison-type, its snake-like body shaped like a Z.
The article also raises the point about the importance of the XYZ axis in the Yggdrasil myth -- and its perfect correspondence with the stag, eagle and serpent. You can see them circled below:
This raises the question: What other Norse myth elements would you like to see incorporated into Pokemon X and Y? For example, perhaps a Vanir trio as the Articuno/Zapdos/Moltres equivalent? And what areas could the myth inspire? Perhaps a chance to explore "Asgard" like Platinum's Distortion Realm? And maybe to find the third serpent, the legendary, we'll venture down below the Pokemon world's equivalent of Yggdrasil, and free the serpent from its roots...