So only mascot Legendaries get any significant discussion, alright. So going down the remaining list:
June: Shiny Zygarde (this is a coin flip. It's related to Xerneas and Yveltal yet it never got its own game (and doubt they'll talk about that). Though considering how complicated its formes are, I'm going to guess one segment will be about how 10% is fast and the other about Perfect's Signature Ability and Move)
July: Tornadus and Thundurus (Probably talk about the differences between their Incarnate and Therian Formes)
August: Kyogre and Groudon (Mascots so high hopes here)
Sempember: Lati@s (Probably talk about their speed/difference in stats and their Signature moves, maybe also mention the Soul Dew but overall not expecting much)
October: Reshiram and Zekrom (Mascots so high hopes here)
November: Lugia and Ho-oh (Mascots so high hopes here)
But that's for the future, what about Xerneas and Yveltal:
Xerneas Creation: Xerneas went through a unique creation process, going to designer to designer with all giving input throughout its creation. Which is rather interesting, as I remember another story about both Xerneas and Yveltal's creation. From Bulbapedia:
"Xerneas, along with Yveltal, was designed by Ken Sugimori and Yusuke Ohmura. Initial design for the two Legendary Pokémon was started by Sugimori, but he encountered "artist's block". For the first time, he handed the work over to another designer, Ohmura, who furthered the designs, after which Sugimori returned to finish them."
Now the two stories aren't incompatible, could be Ken Sugimori started on the two but after having artists block assigned Yusuke Ohmura to further the design. From there Ohmura did this with Yveltal by himself, but since Xerneas was the first Fairy-type he went around the office to the other designers to get their input and design suggestions. After all that was done Ohmura made his version of Xerneas, a combination of suggestions made by committee, and submitted both back to Ken Sugimori to finish them. While "made by committee" is often used as a joke to point out a products generic-ness/pandering, I think it worked really nicely here. The most obvious way to go would have been a traditional fairy-like creature, by making it a deer (probably inspired by the Forest Spirit from Princess Mononoke) they not only curb the stereotype that comes with the word "Fairy" but also give their own definition and idea of what the believe the Fairy-type would be.
Sad Yveltal didn't get any major mention, but could be its design was more straight forward. Opposite of life is death, death is seen as an evil thing thus Dark-type (Evil-type in Japan), it needs to incorporate a "Y" design which looks like a bird (with a long tail), a bird associated with death are vultures, also Norse mythology has a few death birds connected with Yggdrasil, and any other design aspects like the double helix design probably came from Xerneas's final creation so it could match it.
Battle Stuff: Nothing major, they do talk a tiny bit about how Legendary mascots have similar, if not the same, stats but nothing too indepth. They just mentioned despite having the same stats their Abilities give them more unique use (I'd more say that it's their Typing and Move Set which more determines that but whatever you want to believe, GF...). Surprised they made no mention of their Signature Moves, yet another thing which sets them apart as Xerneas is a major buff and Yveltal is a draining move, which is very different from previous Legendary pairs who had matching/comparable offensive moves.