I have the assumption that the Pokemon Z legendary must be a long serpent or dragon similar to dragonair (due to the shape of the letter Z) and since Yveltal is airborne and Xerneas is a land legendary, I would assume this dragon legendary would be a water type of some sort from the sea. Agree? Different ideas?
I think a third legendary as a dragon-serpent of the sea would be cool. I think it would be even better if the sequels (name Pokemon Z and W) would have two seperate legendaries (the sea-serpent and a fire based beastie so we would have an air-earth-fire-water theme going on.)