I'm not sure what exactly to make of Greninja. On the one hand, Rapid Spin being debunked made his potential as a utility poke, which was initially looking like his claim to OU status, take a nosedive, as did news of the newly-buffed Defog, which made his role as a fast hazards setter look a lot less attractive too, as I feel that the ease with which hazards can be removed makes bulky hazard users which have the staying power to set them up multiple times more attractive. However, seeing those damage calcs regarding what it can do with Protean and a Life Orb was sobering. It seems like the ultimate utility-revenge-killer. Its potential position in OU hinges on how much that niche is going to be in demand, I think.
Obviously most of the Megas are going to be sitting pretty in OU, at least the ones which don't end up banned. I think a fair few will be off to ubers. Once you get past those, I think there'll only be a handful of newcomers in OU. A lot of the new Pokémon aren't too great. And y'know what, that's okay. It'll make the lower tiers a lot more interesting instead of OU, and that's fine.
However, that's not to say that there won't be any. The big one is Aegislash. It's possibly the only Pokémon with a mid-battle form-changing gimmick to actually be not only viable but actually extremely strong as a result of it. Its movepool is relatively small, but incredibly focussed. Its an incredible priority attacker after SD in sword stance, it can tank through a ton of stuff in shield mode, it attains perfect neutral coverage in two attacks, and the chicanery you can pull off with King's Shield can be downright cruel. However, I feel like the truly devious battlers might play with the expectations of their opponent a little, and instead of using King's Shield use Substitute instead to deal with the status moves that, as of right now, look like the only sure-fire way to neuter the threat that Aegislash brings to the table.
Goodra I feel is also a shoo-in. It's a strange pokémon with a strange stat spread, but that gargantuan sp. def and mixed offensive options are going to land it on quite a few teams. Dragalge I feel will also make it despite the many claims to the contrary, although that hinges largely on when Adaptability becomes available.
Talonflame and its insanely powerful priority will also give it a spot, I think. Those 80 base attacking stats were almost enough for me to write it off instantly, but I've had to change my mind after seeing what it can do. It's also showing a surprising amount of versatility - I've seen about three or four really solid sets for it, from SD to Bulk Up to even some mixed attacking sets.
At least one of the new fairies is going to make it. Whether that will be Sylveon or Florges or both remains to be seen. They're practically identical pokes when it comes to stats and movepool, but I feel like Sylveon and its (marginally) more useful ability will see that it gets the nod. Of course, there might be room enough for both. Klefki and its prankster shenanigans will also give it a spot.
Zygarde will make it in too, I think. It's easy to point out its shortcomings when compared to Garchomp, but I feel like its mammoth physical bulk, strong typing and its access to Coil, DD, and Espeed for Priority will save it from Kyurem syndrome.
Other than that, I'm drawing blanks. I feel like everything else is outclassed by something else already in OU, and thus will end up in UU and below, which again, I have no problem with because I think it's good for variety. No matter whether I'm right or wrong, I feel like enough is going to change that it's going to feel like a fresh experience, and a very fun new metagame.