Did everyone notice that Tapu Lele didn't get Trick? But yet TAPU FINI did!! Tbh Trick on Tapu Fini is going to be lowkey good, I've already had huge success with Scarf Tapu Fini in THIS SM metagame, as odd as that sounds, and personally will be playing USUM just to try the set with Trick over Defog. It'll arguably be one of its better sets in USUM too, as its very good at beating Greninja and luring things out like Tapu Koko :-).
My thoughts on everything else:
Defog Landorus-Therian is just dumb, as if there wasn't enough reason to use it... I always see Landorus-Therian as evidence that our metagame isn't balanced because no **one** pokemon should garner that much usage, and now it's only going to skyrocket. I can see Landorus-Therian becoming a potential stall Pokemon with leftovers + protect + wish support, since it can now SR/Defog, and Landorus is very good at sponging up attacks from common stall threats in like Mawile & Banded Tyranitar.
Rotom-W getting Defog is actually kind of good, but not sure if Wash will be a huge contender as its bulk is very thin and can only really go into defense or special defense and not both, so it isn't a one-all pivot, especially since dropping Wisp/Split will have big issues in some situations: lack of Pain Split reduces Rotom-W's utility/longevity, while Wisp makes it useless against things it should be beating like SD Mega Scizor. It'll just depend on the team I guess.
Gliscor doesn't really have the room for Defog IMO. Not to say it won't be used or that it would be bad, but it will be competing with Landorus-Therian. Actually now that I think about it, Gliscor could also be a good Pokemon on stall now that it has poison heal + defog legal, although it wouldn't be cohesive with Heal Bell + opposing Knock Off spam.
Toxapex with Knock Off? It can't really drop Scald as it relies on 30% burn rate to be even remotely threatening to things like SD Scizor / DD Gyarados / etc., and it needs Recover and Haze, leaving on Toxic/Toxic Spikes as a replaceable move, but both of these attacks are really outstanding on Pex, so finding room for Knock will not be so easy. Same principles apply to Tapu Fini, as it will likely not have room for Knock Off either.
Y'all talk about Serperior/Tornadus-Therian Defog, but Thundurus getting Defog is probably better thanks to Prankster, though not sure if it will be practical in our metagame when we have Defog Tapu Koko. (I think I saw Tapu Koko Defog mentioned in the pastebin; why is it not listed in the OP wtf Defog Koko will be amazing)
Those Sticky Web users will not be relevant in OU at all lol. Those Pokemon do not see usage at all and giving them Sticky Web will not change this when Shuckle (or Smeargle, I guess) is the best Webs setter(s) anyway. Buzzwole will also still be bad, seeing as how it doesn't really have a strong niche in our metagame. Hydregion getting Defog would have been nice in XY maybe, but uhhhhh, in our Fairy-type dominant metagame you gotta be shitting me to even mention it, because the only niche it ever saw in our metagame was briefly at the beginning of SM OU as a Choice Scarf user, because it's really a sub-optimal Pokemon when you consider how many dominant Fairy-types we have (Koko Lele Magearna etc.).
Thought: Defog-spammy metagame might make Defiant users like Bisharp better, especially if Landorus-Therian becomes a common Defog Pokemon, with Intimidate further buffing Defiant Pokemon.
Gunk Shot Ash Greninja would have been very good early SM OU when Tapu Fini was the only counter to it; now, it shouldn't be very good at all, seeing as it only hits a Pokemon that our community has decided is trash (tip: it's very good, actually). Fini doesn't have that dominant usage anymore, and Gunk Shot really doesn't hit anything (Clefable) that Hydro Pump isn't going to hit harder tbh. Maybe if Gunk Shot was a special-attacking move I could see it being a powerful neutral poison attack, but it's physical, so specs greninja wouldn't benefit from it, you'd probably be running Z Poison.
Heal Bell Magearna isn't even worth mentioning when its best sets are either offensive Z move or the Assault Vest pivot, it just really has no business using Heal Bell ever imo.
-.-.-
Quick question: is two Z moves per battle legal in Wifi battles / legal in competitive Pokemon? Because if it is I'm logging out of Smogon and PS so quickly you have no idea lol
EDIT:
Please do not make any Deoxys form back into OU, it has NEVER worked out EVER!! Same for Genesect, Landorus-Incarnate, and Aegislash THANKS