Er, about the cromossome things...
X and Y are also coordinates of a 2D plane, if I'm not mistaken. If the next game really is Z, it would become the coordinates of a 3D plane. Also, the game is on the 3DS. Think about it.
Anyway, on to the legendaries:
Y (bird) : SWEET MARY JANE IT'S A DEMON oh wait it's just the legendary. Anyway, typing will definetley be Flying/something. Although I REALLY want to believe that something is Poison, i'ts probably going to be Dark. On to the design... It does look quite nice, but I actually like X a bit better. I don't really know why though.
X (deer) : I started laughing a little when it came in. Why? Notice how it walks: It's tip-toeing. I'm naming this guy Tippy if I ever get X. Typing... Hm, I'm honestly at a loss for this guy's typing. I'm getting a Steel/Psychic vibe, but I'm not sure that's the true typing. To the design... Honestly, the less flashy Pokémon always captured my atention, so when I saw it tip-toeing snort calmly through the forest, looking up into the sky majestically, I liked it. Plus, the small X-eyes detail is amazing.
Welp, those are just my two cents...
THERE BETTER BE A POISON TYPE LEGENDARY IN HERE NINTENDO