Just to try and speculate Xerneas's and Yveltal's typing, perhaps maybe we can look at the possible theories that these two are based from. Going along with the theory of genetics and some speculation I saw the web, I made speculation about their typing earlier in this thread:
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?
But now, I'd like to take a look at another theory -- the Norse theory,
So, I'm thinking that "the ability to see souls" might be a clue. Souls definitely remind me Psychic, Ghost, and Dark types. I'm leaning more towards Ghost and Dark for Yveltal, so maybe it can be one of those types; Ghost or Dark, because of it's color scheme of having Dark colors. Then again, Gamefreak might throw a curve ball and call it a "Psychic" type, which can be interesting as well. I think Flying type is a given for it's secondary type. I mean, it has wings and... it flies. Or maybe it's one of those weird pokemon that flies, but does not have a flying type. Although, I find that weird and... not very likely.
But what does this mean for it's typing? Well, this is a stretch, but, the different colors it has reminds me of Beheeyem, so could it be some sort of Psychic type? Then again, Beheeyem is sort of classified as an alien, so, maybe it's colors are just a representation of it's alien status. It's secondary typing can be... tricky to predict. Yuvental is high upon a higher plane, so, I see that Xerneas's secondary typing has to associate on being on the ground. What if it can be some sort of grass type? It can be interesting as it can be assosicated with trees, forests, etc. Although... Celebi is a grass and Psychic type because of associations with Time Travel and forests, so maybe that would be redundant? Or, maybe it's just a single type pokemon? Could it be a steel or rock type, because of Xerneas's possible associations with jewels? That can be quite a stretch, but... yeah. Like Derk said, I'm leaning for it to be some sort rock or steel type myself, which... it's typing when it comes to competitive might be interesting to see, but who knows how attractive it can be.
*oh and was referenced from another website*