So they're blue and red. As we learned from all the mascots from practically Groudon and up, you can't really look at a legend's coloring and accurately guess "oh it must be this," (thank you, Palkia). So we can't automatically think they're any part water or fire simply from that. So nixing those.
Someone mentioned Xerneas "clearly" (hell no it ain't clear) being based on the aurora borealis BUT, while it wouldn't be the first time we've repeated a theme (shout out to Fennekin), Suicune is already our "aurora" pogey. Xerneas looks more like a corrupted "Boreas," which is also relative to Suicune, because Boreas is the "north wind," and who do you think rides those waves?
Hint: it's also blue.
So a reversal might be in order. What if Xerneas is the Boreal Pokémon, that hangs out under them sky rainbows? (What is it doing in a forest?) So where Suicune is just Water, Xerneas might be Ice(/Flying....WIND DAMMIT....or pure flying [ew]). It wouldn't be too far-fetched, since the Nordics have reindeer and are far north enough to have auroras, and if the XY region takes aspects of European countries (and not simply France), you can argue them cold places might too be involved.
But I want to go more definitively with Ice/Steel, because lookit dem sexy-ass legs, mmm boy. They look like fuckin' swords, man. Fuckin' double-edged blades. That shit will cut you up.
Yveltal, on the other hand, looks like a dark dragon from hell. And this is why I want to go more with the dragon theme. It's been pointed out that, in relation to dragons, it's very wyvern-esque (a two-legged dragon with wings where arms or forelegs should be). Which is pronounced y-vern.
[Lugia is vaguely wyvern-esque, too (which wouldn't be the first time a draconic-looking fucker wasn't a dragon) and I personally wish it was an actual dragon thnx.]
Why am I not saying Dark-type? ...Well I want to. Some sort of hellish Dark/Fire/Dragon is damn sexy, but I'm not because I'm operating on a single notion.
What if the starters do grow up to be Grass/Dark, Water/Fighting, and Fire/Psychic?
It's now a perfect round about cycle. And it's only fair to the mascots (which GF seems to have been trying to perfect since Dialga/Palka) that this treatment finally be done to them as well.
Ice slays both Dragon and Flying.
Fighting or Fire slays Ice and Steel.
Dragon is not effective against Steel.
Flying slays Fighting.
If Xerneas is boosted in SDef or Def (depending on Fighting!Att or Fire!SpA), or has an ability that cuts SE attacks in half (x4 to x2, x2 to x1; a stronger version of Filter), it could survive a x4 hit from either fire or fighting. Which gives Xerneas a fighting chance with either a Fighting/Dragon or Fighting/Flying, only having x2 SE STAB.
Stick a Fighting/Dragon!Yveltal with Levitate and boom, genuine dragon.
Those claws make me think it's gonna maul you, so why not Fighting?
It also just seems like the legends of late (back to Gen 4) are just trying different type-combos out, and these aren't on the board yet.
....Or basically I just pulled shit from bumfuck nowhere based on assumptions, and assuming shit is evil.