I will actually try my best to answer this - according to the X and Y Trailer, it shows a little clip of Yveltal and Xerneas. Yveltal is up in the air so I can't say anything about it, but Xerneas may be found somewhere in this area:
THIS ISN'T CONFIRMED.
We don't know anything about wherever this area is - this is just my theory.
Thanks for Serebii for the pictures.
Hope I helped. :)
Necro-Edit lolol: Yeah, turns out I was wrong. :D The legendary of your version can be found in Team Flare HQ - Xerneas for X and Yveltal for Y :D