Ok guys, more theory time. This is one I had for a long time, but Charizard X fits very nicely with it.
So far, all so-called "box legends" represent a higher power in the nature of the Pokeworld. Yes, Ho-oh and Lugia represent renewal and tradition, featuring the main conflict of GS. We know a bit about Xerneas and Yveltal, especially their species (life/destruction), but IMO it isn't immediately obvious how they form a balanced duo and how the expected Z mon will fit in there. So, since most Pokémon (and especially legendaries) are usually based on a mixture of different motives (like the nordic myths we already know Xerneas'and Yveltal's designs are based on, but that have basically nothing to do with the other themes going on there). So long story short, I think that the legendaries this time are going to also be based upon natural evolution, with Xerneas representing mutation, Yveltal representing selection and the third one possibly representing evolution process over time (i.e. ancestry) or something like that. In the end, the evolution theme also ties all loose ends to the game aspects: Evolution happens in DNA, which ties in with the logos, mega stones and the cloning theme. It is driven be the fight between life and destruction (Xerneas/Yveltal species). And its a basic principle of nature that fits the theme of past generations.
Why am I mentioned this now? I think the Mega Stones revealed so far fit nicely with this theme. The Mega Stones Y (both on Mewtwo and Charizard) select the best qualities the Pokémon already have and boost them. The Mega Stones X add new qualities (designwise, typingwise, even battle orientation-wise) to the Pokémon, so they do mutation. Obviously, the revelation of more and finally all mega evolutions and more lore about Xerneas and Yveltal (and possible the third legendary) will confirm or deconfirm this theory in the end, but so far it seems pretty fitting to me.
