First, taking a look at the new pokemon (I won't talk about Greninja, Talonflame, Aegislash, Barbaracle, Sylveon, Hawlucha, Goodra, Klefki or Noivern, which all have fairly solid OU/BL potential as it stands, though some may fall off over time, especially as pokebank/Z version come around):
Chesnaught: With solid attack and defense, an amazing hidden ability that gives him immunity to all sorts of moves for switch in opportunities (sometimes seemingly arbitrarily, but hey, I'll take it) plus access to Bulk Up, Power-Up Punch, Leech Seed in addition to powerful STAB in Hammer Arm and Wood Hammer, Chesnaught has a lot of potential. His low speed and unfortunate typing for a defensive pokemon (ESPECIALLY when stuff like Talonflame is around) make him an unlikely candidate for OU, but I think he has a lot of potential sets in UU. The sleep mechanic changes also benefit him, some Restalk power up punch/wood hammer set could potentially be pretty interesting to play around with.
Delphox: Solid SpA and good speed, unfortunately limited by a mediocre typing and an even more mediocre (offensive) movepool, though it has solid support options. Its hidden ability, Magician, is interesting though. It basically turns every move into thief, and it also gets access to switcheroo. I could see Delphox potentially being a very disruptive presence on a lower tier team, being able to steal critical choice items then later switch them onto enemies. It also has decent special bulk and not-quite-paper-thin physical defenses to boot at 75/72/100 hp/def/sdef, so a more defensive set is possible.
Diggersby: At a glance, it has typical yard trash rodent stats, but it also gets a ground typing and huge power as a hidden ability. With huge/pure power's old main users, Azumarill and Medicham, getting major buffs and seeming likely OU candidates now, this bunny may have himself a niche. It has a pretty solid variety of physical moves, Stone Edge, Earthquake, Hammer Arm, Wild Charge, U-Turn, Return, and Bulk Up/Swords Dance as boosting options. Unfortunately it lacks priority and with 78 base speed, it's not exactly a bolt of lightning.
Vivillion: Compoundeyes Hurricane and Sleep Powder with Quiver Dance is scary, but with its BST, paper defenses, and x4 rock weak typing, it's unlikely to see serious OU play, but could be a significant threat in UU with proper support. Basically a bulkier, faster version of Butterfree with access to Hurricane instead of being able to run tinted lens.
Pyroar: Pretty fast, decent SpA, but with a very, VERY modest movepool and a typing that doesn't bring much to the table, with Normal adding fighting to fire's already unfortunate weaknesses to the common ground, rock, and water type moves. Honestly, I don't even think this poor lion will make UU. It gets Moxie as a hidden ability.... But it has 68 base attack (according to Serebii, some of these stats have been off, but it's clear that Pyroar is a special focused pokemon).
Florges: With high SpA and enormous SDef in addition to a solid support movepool with both wish and aromatherapy as well as the screens AND both rain and sun, Florges looks to be a pretty solid addition to UU, as its modest HP and Speed will likely limit its usage in higher tiers. It does suffer a little from 4 move syndrome but all in all, is a very solid supporter.
Gogoat: Reminds me a lot of Chespin in some ways, a lot of bulk, grass type, but instead of the advantage of a secondary STAB and high defense, Gogoat has the advantages of high HP, reliable recovery in milk drink and balanced offensive stats that make running a mixed set quite viable. He also has the option of Bulk Up which can be scary with reliable recovery. Unfortunately he also has very poor speed and his unboosted defenses aren't too hot. His abilities, Sap Sipper and Grass Pelt, don't seem to bring much to the table either.
Pangoro: It has a stat distribution very reminiscent of Gogoat, minus the respectable SpA with higher Attack instead. His abilities are quite nice, all of them have their uses but I feel that Scrappy will be preferred, as unfortunately he does not get Mach Punch/Bullet Punch to boost with Iron Fist. Otherwise he'd likely be OU material. He does get solid boosting options in Swords Dance/Bulk Up, good coverage (especially important is Poison Jab, which will help him with the fairies that are sure to be floating around the lower tiers) and Parting Shot is an incredible move, but his speed is quite poor and he lacks priority, limiting his sweeping potential. Could be a good Choice Band user, here's to hoping for Bullet/Mach punch tutors in Pokemon Z.
Furfrou: Mediocre stats all around (except for its Garchompesque speed and a respectable 90 SDef), pure normal typing, one might be fooled into thinking this thing was going straight to NU, but its unique ability, Fur Coat, is almost a physically defensive version of huge power and combined with Baby Doll Eyes, makes him potentially the single most goddamn annoying physical wall in the tier. While still lacking the defenses to tangle with most of OU, with access to thunder wave, toxic and roar he can spread status with the best of them, and with the sleep mechanic change, Restalk sets are also viable. He does suffer from a serious case of 4 moveslot syndrome though, and doesn't have a lot of actual offensive moves to scare off taunters, especially those who resist or are immune to Return.
Meowstic: An interesting pokemon because the different gender versions are not only aesthetically different, but also have different hidden abilities. The male is the one to really talk about, as it has Prankster, an ability good enough to help even pokemon as weak as Sableye carve out a niche. Its special attack is modest but it has a pretty solid movepool to take advantage of prankster, with both screens, thunder wave, trick room (though trick room is actually pretty detrimental to Meowstic when it's not getting the prankster boost), and the especially interesting Me First, which protects it from many users of shadow ball, usually psychic/ghost pokemon who are also weak to it. But possibly the biggest takeaway is that with Gardevoir likely moving into OU, Meowstic also becomes the only user of Calm Mind + Stored Power around, which can be pretty threatening especially considering Meowstic's high base 104 speed. I expect Meowstic to be a major threat in UU.
Aromatisse: High HP and both special stats, solid defensive typing, EXCELLENT movepool, and to top it off, the thing is literally slower than Slowbro. That there, combined with its lack of secondary typing for defense against the common Dragon coverage moves, and lackluster physical defense probably spell doom for its overall usage. But it gets Trick Room! Aroma Veil is an interesting ability, I'm not sure what it all blocks, but I'm assuming it prevents confusion and attraction at least. I expect this thing to be a very, very solid addition to trick room teams in particular.
Slurpuff: Mediocre stats and a solid movepool... but honestly, not a lot that jumps out as being particularly exciting. It's pretty similar to Aromatisse except with less abysmal speed and less impressive stats elsewhere, movepool is pretty similar, typing is identical, ability, Sweet Veil, is a little better for the sleep immunity, and it has access to Unburden for some potentially interesting berry sets. But I do not expect the cupcake to do well unless it has some impressive moves we're not aware of yet.
Malamar: Very strong pokemon, it has pretty mediocre stats all around but also Contrary + Superpower, which is basically a power-up punch on steroids, even coming off of 90 base attack, it will hurt. A lot. With STAB coverage from Psycho Cut and Night Slash it has pretty decent offensive capabilities, and the defense boosts from superpower make it a little less flimsy than it would otherwise be (actually, after a boost it becomes quite bulky, on the physical side anyways). Contrary also makes it a potential counter to something like Furfrou, letting it have an easy switch in on Baby Doll Eyes. It also has a pretty solid Special movepool and Calm Mind, but its speed is mediocre, limiting its sweep potential. I could see some Scarf set being an excellent clean up sweeper once all of the fighting resists are removed from a team.
Dragalgae: A very interesting typing, great SDef, solid SpA, but otherwise mediocre stats combined with abysmal speed (and pretty bad HP for being such a slow, defensive pokemon). It brings with it the absolutely horrifying prospect of Adaptability boosted Draco Meteor though, alongside solid, boosted Poison type attacks which will help deal with the faeries who would be immune to said eldritch terror. Might be OU or BL material, but I feel this one's still up in the air simply because of its speed and lack of physical bulk being such a stumbling block for its OU viability.
Clawitzer: Ah, a good ol' fashioned bulky water. His defenses might be lower than others in his weight class, but he also comes with a sky high 120 SpA, and Mega Launcher. Mega Launcher increases the power of Pulse moves by 50%. That brings Aura Sphere to an equivalent of 120BP, taking the nerf into account, Dark Pulse has a similar power jump, Water Pulse hits 90BP, becoming Surf with a confuse chance. He has solid coverage, not a whole hell of a lot in terms of support options, but he's definitely a potential wrecking ball with SpA that high. I'd say he was OU material if he wasn't completely outclassed by Mega Blastoise.
Heliolisk: Classic speedy special glass cannon, electric/normal with access to Dry Skin for a water immunity. Has solid coverage with Focus Blast, Dark Pulse, Thunderbolt, Grass Knot, and even Surf, plus a little utility in Volt Switch. Definitely has some potential for use in OU, but only time will tell if the usage stats reflect that.
Tyrantrum: Solid physical defenses, great attack, a pretty solid offensive typing, GREAT coverage from the fangs and a great ability that boosts them and of course, Dragon Dance as an egg move. I think Dragon/Rock's issues defensively, plus his mediocre base speed will leave him hurting badly in OU, but he has potential to become one of the most threatening sweepers in UU if he ends up there.
Aurorus: A really interesting ability wasted on a pokemon without the stat spread and/or movepool to take advantage of it, an absolutely, irredeemably abysmal defensive typing on a slow, bulky pokemon. I really want to be optimistic but I don't see this guy ending up above NU.
Dedenne: Fairly speedy and has interesting typing, but is lacking in pretty much every other area, its stats are lackluster, its movepool isn't too hot, its abilities aren't very good. Pretty confident that it's NU material.
Carbink: It's the fairy type shuckle, enormous defenses, everything else is garbage. But it has calm mind as a setup option, screens, stealth rock, trick room, sand, hail and sun for support. Don't quite know if it will hit UU/NU.
Trevenant: Though I think he actually has some potential in OU due to being a spinblocker that counters Starmie, we also have to keep in mind that he's competing with Aegislash, who counters Starmie much more effectively. I think Trevenant could potentially end up in UU as a result. He makes a great spinblocker though it remains to be seen whether he'll fare well against defog users of the tier. He has strong attack combined with OK defenses and good HP (for a ghost). Access to Leech Seed, Horn Leech, and a solid physical movepool could make him fairly solid despite his poor speed, and his support options include Will-O-Wisp, allowing him to run a more conventional spinblock set. Oh, and also Natural Cure is a great ability, but Harvest is potentially even better, with Yache/Sitrus berry, or even Chesto berry for the most annoying chestorest set imaginable.
Gourgeist: What a nightmare. When it comes to talking about it, anyways, since it has four different stat distributions. I'd first take a look at the Supersized variant, which has 100 base Attack plus solid HP and Defense. With access to Trick-or-Treat and Shadow Sneak, it could be a very annoying pokemon to deal with, throwing out ToT when it forces a switch, then smashing the poor switch in with a super effective shadow sneak if it doesn't leave immediately. I don't know if it has access to Pursuit as an egg move, if it did this would be doubly nasty. It's not a one trick pony though, with access to, Pain Split, Explosion, Trick, Will-O-Wisp, and Destiny Bond. The small size has 99 speed which might take better advantage of some of its support options like Trick and Destiny Bond.
Avalugg: Well, normally I'd be the first to shit all over a pure Ice defensive pokemon, but with over 180(!) defense with solid HP and Attack, I think it's actually very strong. In fact, I'd say it might even be OU material, despite the confirmed lack of Ice Shard (which would very firmly place it in that tier, in my opinion). It has Rapid Spin as a support option, reliable recovery in Recover and in general, just doesn't give a single fuck. But of course, with a defense like that, you have to offset it in another area, namely in slowbro tier speed. Fortunately he has Avalanche and Gyro Ball to help him spin that negative into a positive.
I think it's too early to talk about what is falling out of favor in OU, so instead, I'll mention one pokemon who will most certainly be jumping up from NU to UU, or possibly even BL: Exploud. STAB Scrappy Boomburst, there is literally no escape from that terror.
