So Xerneas is the "life Pokemon" and Yveltal is the "destruction Pokemon". What could the third legend be?
Preservation Pokemon. I am Hindu. We have a trinity thing: Brahma is the Creator, from whom life springs forth. Vishnu is the Preservor, whose eternal drumbeat reverberates with the rhythm of the universe. Shiva is the Destroyer, who brings all things to an inevitable end.
Xerneas is Brahma, Yveltal is Shiva, and the third legendary is gonna be based on Vishnu. The story will be about finding the balance between creative and destructive forces.
But hey. This is just a theory. A game theory.
sumuthergamer = some other gamer. Get it? Call me "gamer", not "sumu"...
If you believe the wiki, this gen seems to be based on norse mythology. Yvetal is based on the guardian who protects the top of yggdrasill, Xerneas protects the trunk and that means the third legendary may be the snake that dwells beneath the tree. This suggests to me the theme seems to be heaven, earth and the underworld.
The idea has crossed my mind. A lot of people have brought up connotations to Norse myths too, based on what appears to be a huge tree in the Northeast corner of Kalos.
The idea is that the tree is Yggdrasil, Xerneas is one of the four guardian stags, and Yveltal is the eagle. I personally think Yveltal leans more toward Niddhoggr, the dragon who gnaws at Yggdrasil's base and is associated with death and destruction, but the reveal of the third legendary will show us how that works.
Plus, France is partially famous for the Normans, descendants of the Vikings.
That is not dead which can eternal lie, And with strange aeons even death may die.