Regarding Sun and Moon themselves, I'm actually becoming increasingly convinced that Zygarde will play an important role in these games along with Xerneas and Yveltal themselves. Zygarde might become a very prominent figure in Sun and Moon's story, and this generation will almost definitely be a GSC esque game to X and Y and will be connected to Gen 6 in some way or another. Also, speaking of which:
Feast your eyes on Perfect Zygarde, everyone! We saw this long ago, but look at its chest. ITS CHEST: TAKE A CLOSE LOOK. Look closely, and you shall see: Pokemon Sun and Moon is evident there. Pokemon Sun and Moon's color schemes are red-orange and dark-light blue. Zygarde's chest has Pokemon Sun's red-orange color and Pokemon Moon's dark-light blue-white color as a line across its own chest! This is in addition to its appendages being X blue and Y red, but Pokemon Sun and Moon's color schemes are there, right on Perfect Zygarde's chest. I could be wrong, but I think this is proof that our beloved Zygarde formes will play a strong role in Gen 7 and Pokemon Sun and Moon. Zygarde is highly likely to be extremely prominent to Sun and Moon's story from my perspective. I think we shall see Thousand Arrows and Thousand Waves, as well as AZ's Floette, enter fruition in Sun and Moon as well. Perfect Zygarde is becoming a piece of evidence that we may see Sun and Moon have a strong connection to X and Y, and it's possible that Kalos will be in Sun and Moon as well, even if we don't see it as the primary region.
So yes, I'm certain Zygarde is in the same level of legendaries as Xerneas and Yveltal as well as all the other box legends. It's no ordinary legendary Pokemon like those bird or dogs. And it will be storyline relevant in these games, I'm sure. Thousand Arrows, Thousand Waves, Light of Ruin, and AZ's Floette might actually become legal here in Gen 7, as well as the Zygarde formes too.
That's just my thoughts regarding some of this.