The more I think about it, the more a Mega Zygarde or two makes sense. A Mega boost would be enough to put it at 700 BST, which would equal the BSTs of Black and White Kyurem, making it strong enough to plaster onto box art (as well as probably less hideous). It's already got connections to Mega Evolution by virtue of its connection to Xerneas and Yveltal; it's hypothesized by Sycamore that the box mascots' energy channelled through the ultimate weapon is what mutated Mega Stones into existence, and exposing the box mascots' energy to the Mega Ring is part of what allows the Mega Ring to be upgraded. Even Zygarde's body seems to be related to Mega Evolution; its tail resembles base pair codons (the so-called "rungs of the ladder" of the double helix of DNA), and if the people smarter than me over at Bulbapedia are to be believed, the patterns on its body are similar to those of ribose and deoxyribose molecules.
There's also the implication in Zygarde's Dex entries of it being a guardian of the land; however that role ends up manifesting itself, it's got a secret power that it will reveal when it does, so something's gonna happen to it.