...I've leafed through a majority of this thread over the past few days, and the reacurring theme of Xerneas/Yveltal's types has really intrigued me. This is my first little post on this site, so forgive me if anything's off.
I'm getting the feeling Game Freak's going to toss us a curveball on what types these guys are--especially Xerneas. After what they pulled on earlier Legendarys (Lugia, Reshiram and Zekrom instantly come to mind), I'm afraid to make any concrete assumptions. For now though, I'll assume Yveltal's at least part Flying, and Xerneas is...er...well, Xerneas.
Not against new types, but at the same time I'm not really for it either: Sound as a type doesn't really seem that appealing, and we all remember what happened to Reshiram, who looked like a "Light Type" to a lot of folks.
I'd rather they rebalance the cast of Types already in use before they add more to the pot: Grass really got the short end of the stick defensively, and most players I run into neglect them entirely unless they have secondary typing to take the sting out Grass's weaknesses--see Ferrothorn. (That, and make every dual type possible too--I'd kill to see the looks on player's faces if they make a Dragon/Fighting type with mixed offenses.)
While I admit Light seems like a good idea (SHIELDS HIMSELF FROM ANTI-NEW TYPE POSTS), I doubt they'd debut it on a Legend and call it a day... Xerneas gives off more of a "Steel-Type" aura than anything. How would Hidden Power work if new Types were added now?