I'm not sure if anyone has seen this, or have speculated this, but I saw an interesting theory by some fans on the web:
Xerneas and Yveltal look very organic.
Xerneas might represent a Neuron, Yveltal might represent Muscles and Blood.
That means 2 of the 3 germ layers are covered.
Almost all animals develop out of 3 layers.
The third missing layer is the Endoderm.
The third legendary will represent the Endoderm.
Hm, if that's the case, and if we're still going along with the whole 'biology' theme of this pokemon game, perhaps these theories can shed a little light on what their types might be. Well, look - A neuron is an electrically excitable cell that processes and transmits information to the brain through electrical and chemical signals. So, Xerneas, representing that 'neuron' could potentially be an electric type! (with the key word 'electrically'). As for it's secondary type? Well, that can be in the air; it can be anything, imo. One of the types I'm leaning more towards is grass, because, well, I'm thinking that the trailer provided a sort of hint that it might be a grass type, because it was conveniently in a forest. Then again, it could also be a steel type, because its art sorta... 'looks' resembled from Dialga, maybe it can also be Psychic, since it miiight fit in with it's natural ability of making it's horns glow vibrant colors, or with this possible neuron theme.
As for Yvental, I guess from the theory that it can represent muscles and blood, could it be some sort of Dark or Water type? Well, even though blood is thicker than water, it -is- still technically a liquid. Then again, maybe it can be a poison type too? Since 'poison' can be an adaptable substance like toxic chemicals. As for muscles, well, fighting types pretty much resemble muscles, so could that be a possibility? Then again, I'm also leaning more towards it being a Dark / Flying type as well, from my primary... speculations.
Maybe their types can be something like that?