About the precise point of the BST cutoff, I think 710 is a good spot for it. Most things around that tier have better offensive than defensive stats, especially once the 700BST megas like Garchomp, Tyranitar and Diancie are considered. With that in mind, I think allowing the use of Zygarde-C would be a decent defensive counterweight, and I wouldn't be too worried about it being dominant given that Pixilate Diancie-M and Refrigerate Kyurem forms are likely to be viable options. (For the same reason, I don't think Giratina alone is likely to hold defensive archetypes together; its dragon typing is too much of a liability.)
720, on the other hand, means that we're likely to see a *lot* of teams which are just two Arceuses, and I don't think it would add anything on terms of type diversity because nearly every type is well represented at 710 anyway. (The only ones I can think of which are missing within 40 points of that cutoff are Grass and Bug, and Grass is likely to be covered by Calyrex.)
720, on the other hand, means that we're likely to see a *lot* of teams which are just two Arceuses, and I don't think it would add anything on terms of type diversity because nearly every type is well represented at 710 anyway. (The only ones I can think of which are missing within 40 points of that cutoff are Grass and Bug, and Grass is likely to be covered by Calyrex.)