Here are my initial thoughts based on just the stats, moves, and ability texts. I haven't seen any hard numbers on the new abilities, so I'll be making some guesses. I'm ready for all these takes to age poorly, but if even one pans out I will point to this post as proof of my peerless genius :)
Busted On Paper
Another generation leak, another month of people freaking out about stats of Pokemon they haven't played a single game with. That said: there are some Pokemon packing a lot of power in this generation. Will they pan out to be actual meta threats? Here are the early ones to watch:
Roaring Moon - 105/139/71/55/101/119 (Total: 590) - Dragon/Dark - Protosynthesis/Protosynthesis/Protosynthesis
Based purely on stats, Roaring Moon has the most splashable potential. It has the best blend of power, speed, and bulk of any of the Paradox Pokemon. For reference, it is stronger and faster than Garchomp, has more Special bulk, and a little less Physical bulk. It can setup with Dragon Dance, support with Tailwind, or pivot with U-turn. It pays for these advantages with less optimal typing, Terastylizing could mitigate this but it can probably operate fine without it. Overall, it seems worth using on aggressive balance cores, even completely ignoring the ability. But factoring in the ability is where things go from "good" to "potentially meta defining".
I disagree that Roaring Moon can't fit on a Sun team. All the meta defining Sun teams of last generation were Trick Room optional, flexing GMax Charizard as a powerful sweeper. And I think there is a real potential for Roaring Moon to slot into Sun teams in a similar role. It threatens a lot of Pokemon that match well into typical Sun leads, like other Dragon-types or Fire-types. And if Protosynthesis is a 1.5 level boost, wow are these calcs crazy. It pretty much 2HKOs anything that doesn't resist its STABs.
+1 252 Atk Life Orb Hydreigon Crunch vs. 252 HP / 44 Def Rotom-W: 126-149 (80.2 - 94.9%) -- guaranteed 2HKO after Sitrus Berry recovery
+1 252 Atk Life Orb Hydreigon Crunch vs. 252 HP / 4 Def Corviknight: 133-157 (64.8 - 76.5%) -- guaranteed 2HKO after Figy Berry recovery
And if you want to Tera to Fire-type, well you basically OHKO anything that doesn't resist, and 2HKO even moderately bulky resists.
+1 252 Atk Life Orb Fire Tera Type Hydreigon Tera Blast (Fire) vs. 0 HP / 0 Def Ground Tera Type Garchomp in Sun: 220-259 (120.2 - 141.5%) -- guaranteed OHKO
+1 252 Atk Life Orb Fire Tera Type Hydreigon Tera Blast (Fire) vs. 252 HP / 4 Def Rotom-H in Sun: 99-117 (63 - 74.5%) -- guaranteed 2HKO after Sitrus Berry recovery
+1 252 Atk Life Orb Fire Tera Type Hydreigon Tera Blast (Fire) vs. 220 HP / 172 Def Cresselia in Sun: 157-187 (70.4 - 83.8%) -- guaranteed 2HKO
Palafin - 100/160/97/106/87/100 (Total: 650) - Water/Water - Zero to Hero/Zero to Hero/Zero to Hero
As far as we know, the only thing Palafin needs to hit the above stat spread is to hit the field, switch out, and come back in at somepoint. Lead Palafin, Flip Turn or hard switch out, then bring it back after a slow pivot or KO to clean up. If Roaring Moon has solid balanced stats, Palafin's are something else. Acceptable bulk, similar to Roaring Moon, mid tier Speed, and absolutely ridiculous Attack. Now needing to switch out is somewhat exploitable. If an opponent takes aggressive moves in the early game, taking advantage of the weak Flip Turn or switch turn, even a strong Pokemon in the end game might not be able to make up the deficit. Still, its power is undeniable and coverage is strong.
252 Atk Life Orb Wishiwashi-School Liquidation vs. 252 HP / 4 Def Corviknight: 105-125 (51.2 - 60.9%) -- guaranteed 2HKO after Figy Berry recovery
252 Atk Life Orb Wishiwashi-School Ice Punch vs. 0 HP / 0 Def Garchomp: 270-322 (147.5 - 175.9%) -- guaranteed OHKO
252 Atk Life Orb Wishiwashi-School Close Combat vs. 252 HP / 44 Def Rotom-W: 95-112 (60.5 - 71.3%) -- 89.8% chance to 2HKO after Sitrus Berry recovery
- 80/120/80/90/65/135 (Total: 570) - Dark/Ice - Sword of Ruin/Sword of Ruin/Sword of Ruin
- 55/80/80/135/120/100 (Total: 570) - Dark/Fire - Beads of Ruin/Beads of Ruin/Beads of Ruin
No calcs here because I don't know if these abilities are stat drops or static field effects, and if its the latter the question becomes "how much does it debuff?" Regardless of the level, I see these both as prominate early meta threats. They might be significantly more frail than the first two I discussed, but their debuffs helps not only themselves, but also their partners (though watch out for the reduced partner defense!). Chien-Pao has STAB priority with Ice Shard/Sucker Punch, while Chi-Yu threatens STAB spread with Heat Wave/Snarl.
Game Freak went HARD with pivot moves this generation. That is exciting after a generation with only one or two good pivots (you know the ones). I think in general, pivots increase the skill cap of the game, as players get access to plays that dynamically change the state of battle. They also encourage switching by decreasing the opportunity cost of wasting a turn to switch. Let's see what we got:
Priority Parting Shot
Both Grimmsnarl and Grafaiai get access to Prankster and Parting Shot. Off the bat this seems like ridiculous support; drop an opponents stats before they can even move, soften the blow for whatever is coming in, and enjoy your positioning against a weaker opponent next turn. I definitely think these will see a lot of play, but there is a fair amount of counterplay here as well. Dark-types (of which there are 3 in the above section), Psychic terrain, priority blocking abilities, the new Clear Amulet item, new Pokemon with Clear Body and other status blocking abilities, new Defiant Pokemon. All these punish spamming Prankster Parting Shot, and will certainly rise with Grimmsnarl and Grafaiai.
We have two Shed Tail users: the fast Cyclizer (base 121 Speed) and the slow Orthworm (base 65 Speed). Both have healing, Cyclizer through Regenerator and Orthworm through Earth Eater (absorbs Ground-type moves for HP). Both have a great niche: Cyclizer to bring something in safely without taking damage itself, and Orthworm to tank a hit and bring in another Pokemon with an intact Substitute. This safe switching can give teams with frailer Pokemon options to change positioning without taking direct hits.
U-turn, Volt Switch, Flip Turn
All these moves got even more users, including many Paradox Pokemon. Nothing really new here, just expect these moves to return on Assualt Vest and Choice sets while they were absent on many Dmax meta sets.
Now this is a weird one. A move that sets Snowstorm and switches out. As far as we know now, its exclusive to Slowking and Slowking-G. This could be an interesting tech to clear weather, and manuaver an Ice-type into battle to take advantage of its increased Defense and Blizzard spam. There are some good Ice-types this generation, maybe with the buff and the option to Tera to a better defensive type will make them viable.
Dark Horse Picks
Okay maybe its a little early to be calling things "underrated." But here are some things I think we could surprise us in a top cut appearance:
Brute Bonnet - 111/127/99/79/99/55 (Total: 570) - Grass/Dark - Protosynthesis/Protosynthesis/Protosynthesis
Okay the typing is much worse than regular Amoonguss. Weakness to Fairy, Fighting, 4x to Bug, and lack of Regenerator hurts its logevity. However it offers something Amoonguss does not: offensive presence. Its not limited to Sporing and waiting around for opponents to wake up. A 127 Attack stat is good on its own, and it can be paired with Torkoal on Trick Room teams easily to get that extra boost.
Iron Bundle - 56/80/114/124/60/136 (Total: 570) - Ice/Water - Quark Drive/Quark Drive/Quark Drive
With a great Speed and Icy Wind, it can act as a budget Regieleki. If you actually get Quark Drive active, it can even surpass Regieleki in terms of Speed (though this seems difficult). It can pivot with Flip Turn, has acceptable Defense that actually becomes bulky with a Snowstorm boost. Strong enough Special Attack to spam Blizzard. Its got plenty of tools, we'll see if there are builds that it can shine on.
Dondozo - 150/100/115/65/65/35 (Total: 530) - Water/Water - Unaware/Oblivious/Water Veil
How it works. Silly gimmick or the next Coalossal? +2 to all stats, Oblivious to guard against Intimidate drops, and can boost its Attack more with an 80BP attack, but it has to fight 2v1 until it faints. In the wrong situation, it can become setup bait, unable to do anything while an opponent positions an endgame. In the right situation, it can use its effective base 121 Speed and good movepool to pick up a couple KOs while sponging hits.