While not every mon past the 600 BST threshold is OP, I feel like there are so many massive threats that a ton of mons would end up getting banned anyway. At the very least, mons we currently have now with one sky-high attacking stat (Sciz, Hera, Pin, Zam, Gar, Kart) have a pretty crappy Atk/SpA stat to compensate, and mixed mons with high offensive stats like Blace, Deo/Deo-A, and Phero have paper defenses (the only one that doesn't is Genesect, who I don't really see a whole lot of for some reason).
If we were to step over the 600 limit, I think 625 would be a good place to start. For that we'll get:
Mega Lucario
Mega Venusaur
Mega Ampharos
Mega Gallade
Mega Gardevoir
Mega Aerodactyl
Mega Steelix
Probably missed one somewhere
I feel like these mons would fit in nicely, however potential problem mons I can see are M-Luc (Steel typing, high mixed offenses of 145/140 with good bulk, can't be poisoned), and M-Venu (huge 80/123/120 bulk, good 100/122 offenses, can't be poisoned, would probably be very hard to take down on a Flower Veil team).
In the 630-640 range you start getting things like the M-Zards, M-Blaziken, M-Aggron, Ash Greninja, M-Gyarados, M-Sceptile, M-Swampert, etc some of which I feel would be a little too intense to suddenly bring into the meta, which is why I feel 625 would be a good starting point.