Very interesting theory. However, I'd say that the most compelling explanation for the origins of Generation 6 and speculation for the hypothetical Pokemon Z legendary comes from the theory that this generation has its roots in Norse mythology.
The whole thing is explained succinctly (if a bit crudely) in this Youtube video:
http://www.youtube.com/watch?v=r-OMbaRBvOw
I think this theory is incredibly cool. I love all sorts of mythology, by the way, so...yeah. I love guessing the myths from which the creators derived the legendaries of each generation.
Essentially, evidence strongly suggests that the legendaries of Pokémon X and Y are derived from the Norse myth of Yggdrasil, the giant cosmic tree that connects the nine realms (Earth/Midgard being one of them, and Asgard (Norse heaven) being another). Anyway, Yggdrasil is inhabited by several legendary creatures. Chief among these creatures are the three following:
- A blind eagle perched at the top of the tree who can peer into peoples' souls
- Four stags (deer) who live in the branches of Yggdrasil and have gemstone encrusted antlers, each with a different color
- An evil, serpentine dragon who lives beneath Yggdrasil and gnaws at its roots
So essentially, Yveltal is based on the eagle and Xerneas is based on the stags. Therefore, its highly likely that another legendary, one for Z, is based on the dragon who lives beneath the tree. And it really fits: who better to have a Z-shaped body than a serpent?
