Here's a roundup of some of the changes we can expect from this gen.
New:
Assault Vest - There are a lot of potentially great users of Assault Vest in the tier. Although losing status, hazard, recovery, and setup moves is a huge tradeoff, there are many that would rather have the defensive boost I think. Palkia springs to mind as the prime candidate for this item, since it often uses 4 attack sets anyway and it further improves its ability to take on Kyogre and even opens up the possibility of checking CM Arceus (Dragon Tail). If Defog renders Stealth Rock less desirable, Dialga and Tyranitar also seem like great Assault Vesters. I could see even Yveltal forgoing Roost in favor of more immediate bulk to more effectively check Arceus, healing up with Oblivion Wing instead.
Shadow Tag Mega Gengar/Chandelure - These two pokes seem poised to revolutionize the way the tier is played. Anyone that has played DW Ubers knows how insane Chandy is there. Although the Drought nerf this gen could prevent sun from staying up too long, it shouldn't be a problem for Shandelure. Mega Gengar is rather similar to Chandelure. Although it loses out on items and a sun-boosted stab, it makes up for it with huge SpA/Speed and stab sludge bomb, which suddenly became relevant this gen with the likes of Xerneas and Fairy Arceus running around. It is also important to note that both of these pokes can serve as checks to SD Arceus forms since they outspeed and are immune to Extremespeed. These two pokes also make a lot of trapping and U-turn strategies very effective.
Yveltal - Although Ghost Arceus has gotten a huge boost this gen, Yveltal could serve to counteract all of those buffs. Whether it decides to use Roost (yay!) or Assault Vest + Oblivion Wing, it will have solid defenses and good recovery. It appears that Stealth Rock will be more difficult than ever to keep on the field, which is only a good thing for this mon. Not getting Pursuit is a dagger, as that would have given it additional utility as a trapper. Yveltal's release also coincides with Steel's Darkness resistance being revoked (which Flying Yveltal clearly welcomes) and Fairy typing providing a new Darkness resistance. Unlike Darkrai, Yveltal has a secondary stab that really helps wreck those Fairy types.
Xerneas - There has been a lot of hype recently about Geomancy Xerneas, but I think it is mostly unfounded. Yes, the mere existence of such a powerful threat could lead to an increase in (Mega) Scizor/Lucario usage. However, Mega Gengar and the aforementioned pokes among others (Arceus Poison/Normal) will serve as a huge road block to potential Xerneas sweeps. Spending an entire turn doing nothing in a metagame full of the most powerful threats ever seems like a risky proposition. The mere existence of Fairy types in the tier could lead to a reduction in the amount of Dragon Tail in the metagame, which is a welcome change in my opinion. Xerneas (Especially an Assault Vest variant) also serves as a check to Darkrai once sleep clause is active. Sleep Talk might be a staple on choice sets. However, the existence of Fairy could also lead many pokes to try new coverage options. For example, Dialga and Darkrai might adopt Flash Cannon (more likely) and Sludge Bomb (maybe not). In general, I think that non-Geomancy sets are being severely underrated while Geomancy is very overrated.
Fairy Arceus - Part of the reason that some Xerneas sets are underrated is that Fairy Arceus will do non-Assault Vest defensive sets much better than Xerneas thanks to Recover. Fairy has a lot of good coverage options, so it will be interesting to see what moves SD/CM Fairy Arceus choose going forward.
Mega Mewtwo X and Y - At first glance the new formes seem insane, but on second thought I'm not sure that Mewtwo has the physical movepool or needs the speed boost enough to welcome these additions. Life Orb could very well be better, but time will tell. The reduction in physical bulk could be bad for Y, but one Life Orb recoil probably has the same effect. Insomnia is pretty nice, but you would have to have Mega Evolved already and Mewtwo isn't really known for switching in easily. I expect Mega Mewtwo Y to be the one that makes waves but 194 Attack shouldn't be underestimated.
Defog boost - Defog Scizor and Latias seem very viable at first glance, but I don't think that the learnset has been entirely confirmed. If this has any kind of decent distribution the change will resonate in the Ubers tier, which has long been dominated by hazard stacking + ghost combinations. Magic Bounce is rumored to stop defog now. It is possible that the spinner/ghost game becomes a thing of the past and is replaced by a Taunt/Magic Coat/Magic Bounce defense but that seems like a losing battle. It seems more likely that the significance of hazards will just be greatly reduced.
1 Kyogre - has nerfed rain and Assault Vest creates new rivals, especially improved Palkia and Dialga among others. The rain nerf doesn't have an effect on the way Choice Ogre variants are played, but it does greatly affect team building as Swift Swimmers become a lot more shaky. In general the weather war seems much less significant, although five turns is a lot of time in a tier this powerful.
2 Arceus - It appears that Normal Arceus usage can only suffer from the developments of this gen due to Chandelure and Mega Gengar revenge kills and potentially Aegislash making coverage more difficult. However, if Xerneas turns out to be the real deal Ekiller will be an important revenge killer. This might be a silly thought, but Chesnaught could check Ekiller decently as well, although I doubt it can kill it in one hit without prior damage.
3 Mewtwo - The potential for mewtwo to have 194 attack or special attack is quite scary and makes Mewtwo all the more difficult to switch into. He will continue to be a huge player, but might be kept down by all of the revenge killers aimed at Xerneas.
4 Darkrai - This generation is definitely a net negative for Darkrai. The new neutrality of Dark on Steel does not counteract the new (old?) sleep mechanics or the introduction of a type that walls Dark/Fighting. I expect an increase in the number of Sub Darkrai and a reduction in the Nasty Plot + two attacks variants.
5 Groudon - Groudon likely got better this gen. Assault Vest is a great tool that will allow it to match up better against the likes of Dialga. Although Drought suffered the same nerf as Drizzle, there was a lot less Chlorophyll in the metagame than Swift Swim, and Kyogre tended to survive longer anyway, so it is nice that sun teams won't have to worry about permanent rain ruining their day. Defensive Groudon variants will likely rise in usage since they act as mediocre but necessary checks to SD Ghost Arceus.
6 Palkia - I've said this a lot already, but Assault Vest is a huge boon to Palkia. It always wants the ability to bluff a choice scarf, but Haban Berry, Expert Belt, and Lustrous Orb are not that enticing. As a Dragon that has a great secondary stab with which to hit Fairies, Dragon Tail might be an option to utilize Palkia's new bulk.
7 Genesect - Losing two resistances hurts, no doubt. Although this is still one of the best revenge killers in the metagame, Xerneas will put a lot of pressure on people to slot Scizor instead, and Defog could be too harmful to momentum-based strategies. The introduction of Shadow Tag Chandelure into the metagame also creates a lot of tension for Genesect. It can't be caught using a coverage move if Chandy is alive.
8 Dialga - Say hello to one of the only Dragon types with a secondary typing that will make Fairy types think twice before switching in. Assault Vest is a welcome addition for Dialga, as it often relied on its resistances and bulk. Flash Cannon slots nicely into the place held by Stealth Rock in previous gens. Speaking of resistances, this is another mon that loses out on a Ghost and Dark resist. It already couldn't switch in on Focus Blast Ghosteus variants or SD Ghost Arceus, so the change isn't that big of a deal. However, it can no longer afford to switch into Judgment/Fire Blast variants.
9 Lugia - If Defog lives up to the hype, this is one of the best pokemon to take advantage of a rockless field. However, SD Ghost Arceus is poised to be the most prominent physical threat in the metagame, and that Ghost weakness hurts. Mega Gengar and Chandelure are also new Ghosts coming to the party that make Lugia's life hell on the special side, so Lugia is probably not poised for a comeback this gen.
10 Rayquaza - Again, this mon really appreciates a hazardless environment, so it will be interesting to see if Rayquaza sets evolve to take advantage of the new metagame, perhaps using more Draco Meteor sets. The Fairy type REALLY hurts Rayquaza, as it was fond of spamming one Dragon attack or another and doesn't have a great coverage option for Fairies either.
I'll analyze the rest of the top 50 Ubers in the new metagame when I get back home.