Also everyone will probably laugh hysterically at this ,but the fact Yveltal physically looks like the shape of a Y, and the little resemblance to the letter X (barring the eyes and the tips of the antlers being identical to those of the tips of the letter X in the title of pokemon X ).
I would like to see Xerneas’s antlers seperating down the center in half almost to make the letter X. It could be used as a ram of the center of the X then could be used to fire up some sort of beam like attack. ( use imagination its hard to describe sorry )

Yveltal I hope is not dragon. A dark/flying type seems logical for such a vicious looking pokemon based on an eagle.

Heres hoping that the 3rd legendary ( Z ) is based off the Serpent / Dragon to complete the norse mythology , and we get a squirrel ( mew, celebi etc ) legendary aswell 😀

I like your idea about them representing Life and Death, i think they might represent Peace and Chaos. Xerneas being Peace and Yveltal Chaos, i think there could be conflict between the two, in the trailer we got maybe Yveltal is coming from the skies to cause trouble to the world and Xerneas is appearing to confront it ?

Xerneas – Psychic/Steel : I think it might be Psychic because of the multicolored “gems” on its antlers. And Steel because of its colouring and how its legs look swordish XD (I sometimes forget Lucario is part steel)

Yveltal – Dark/Flying : Im not really getting a Dragon feel from it, i dont know why i just think its more like a Demon Vulture in my eyes XD. Heck it might even be Poison, i dont think we’ve had a Poison legendary Pokemon yet, have we ? Well that might be a type for the third Legendary that goes with these two if there’s one XD

The best way to balance them would be Poison/Flying for Yveltal and Grass/Steel for Xerneas. (C’mon it has a tree on its head and swords as feet). And despite poison and flying being super effective against grass, Steel would make Xerneas resistant against its counterpart. what do you think?

But the simple fact that there are so many moves of these “types” makes it more troublesome. Granted there were a few moves that changed type from Gen 1 to Gen 2 (and Curse from Gen 4 to Gen 5, but that’s for different reasons entirely) but if they were to do it for Light and Sound, there would simply be too many. And it’s not even as though they’re in just one type to start with, though some not as strictly as others. Examples of moves that either explicitly or intuitively employ Light and Sound:

Light:
Aurora Beam
SolarBeam
Confuse Ray
Light Screen
Reflect
Sky Attack
Flash
Morning Sun
Moonlight
Sunny Day
Mirror Coat
Luster Purge
Tail Glow
Doom Desire
Mirror Shot
Flash Cannon

Sound:
[Everything affected by Soundproof]
Roar of Time
Synchronoise
Heal Bell (no longer affected by Soundproof)
Lucky Chant

Some are able to be disputed, I’m sure, but the point is that it’s not just 4 moves being changed, it would be dozens. Unless they really wanna throw things for a loop, this isn’t a good idea.

Xerneas – Grass/Electric
Yveltal – Dark/Flying