So I'm going to try to guess at viability ranking positions of the new Pokemon and Doduo, who has decided to turn over a new leaf in gen 7. Keep in mind that all of my predictions assume a metagame similar to late ORAS, which has a good chance of not being true given any new mechanics.
Cutiefly
I feel that most of the newly introduced Pokemon will simply be adapted into the metagame, and at best find a decent but not particularly game-breaking niche. But not Cutiefly, who has the tools it needs to help shape the metagame as one of LC's top threats. Quiver Dance + Baton Pass is its main draw, as Cutiefly naturally synergizes well with powerful Special attackers such as Porygon, Magnemite, and Skrelp to act as an incredibly threatening quickpasser. Despite middling statistical bulk, its fantastic Speed lets it invest heavily in its defenses, which, coupled with numerous important resistances, will certainly allow it opportunities to set up. Cutiefly also has some tricks like Reflect, Aromatherapy, and Roost to potentially pass off multiple boosts, which, if successful, would probably mean GG.
But it doesn't stop there. A Life Orb set could in theory be quite strong, as it allows Cutiefly to get around Foongus and Ponyta, usually OHKOing with a +1 Psychic or HP Ground after Stealth Rock, and grants it the power needed to sweep frailer teams on its own after a single boost while outspeeding almost every relevant Choice Scarf user (for reference, Modest Cutiefly has the same Special Attack as Timid Elekid before a boost, and ties with Scarf Gastly at +1). Thankfully Cutiefly doesn't learn Stored Power, but it should still have plenty of utility on Baton Pass chains due to its great defensive typing and, of course, access to Quiver Dance. The nerf to Gale Wings only serves to make Cutiefly more difficult to deal with. Even Life Orb or various coverage moves might not be necessary - many of Cutiefly's best potential checks are trapped by CM Gothita, which is still on the rise.
Predicted ranking: S/A+
Doduo
This is probably the single biggest direct buff SuMo has given us. Currently, Doduo only has a few decent checks, but can otherwise generally be dealt with by simply wearing it down. Jump Kick is absolutely fantastic in breaking through Doduo's already rare checks, and additionally has solid neutral coverage with Knock Off so that Doduo has to use Brave Bird less. It still competes with Aipom as a frail wallbreaker, but Quick Attack and an immunity to Ground are enough for Doduo to hold its own. On the other hand, with Fletchling being nerfed, Doduo will probably be the premier sweeping bird thanks to its newfound access to Swords Dance, which is still an important role since Vullaby isn't going anywhere. I imagine Eviolite or Berry Juice would be preferred on these variants as Doduo would be quite difficult to set up otherwise, and even without Life Orb, +2 attacks off 18 Attack hit very hard. However, if it does manage to get +2 with Life Orb, even Scarf users will be hard-pressed to stop it, as a boosted Quick Attack OHKOes Scarf Mienfoo after Spikes.
Predicted ranking: A
Alolan Grimer
This is basically Stunky but better (minus Defog) because it fixes by far Stunky's biggest drawback: a lack of offensive presence. Between Poison Touch Knock Off and Poison Jab (or non-Poison Touch Gunk Shot) coupled with up to 18 Attack, Grimer has no such issues. At minimum this should be B- since it improves on Stunky, but being able to all but guarantee the removal of important Pokemon in Gothita, Gastly, and Abra while still holding its own against other matchups by actually being quite difficult to switch into should give it a good niche, though Diglett prevents it from being top tier. If Timburr becomes a dominant sweeper again (though I doubt this will happen while Cutiefly is around), Alolan Grimer would likely become meta.
Predicted ranking: B+
Mareanie
The gaping weakness to trappers certainly hurts it, but Water/Poison is still one of the most useful defensive types in LC. Coupled with Regenerator and Recover, Mareanie renders RestTalk Skrelp all but obsolete. I feel that this will overthrow Omanyte as the go-to Toxic Spikes setter as it can set them up throughout the match. As a defensive Pokemon, Mareanie isn't hindered by trappers in the same way as offensive Pokemon like Salandit, because it doesn't spend as much time trying to KO the enemy, so Diglett and Scarf Gothita have much more trouble getting in. CM Gothita completely shuts it down, though. Even with its vulnerability to trappers, the defensive utility Mareanie offers when paired with Trace Porygon (and another trapper lure, so that Mareanie isn't trapped first) or simply against teams without trappers should give it plenty of use.
Predicted ranking: B+
Mudbray
Bulky attacking Ground-types so far haven't been super great in LC because the types they resist tend to be used by Pokemon with adequate coverage moves to get around them, and low Speed limits them from abusing the strength of Ground as an offensive type. This means that in general, slow Ground-types don't have many switch-in opportunities and aren't that threatening once they're in, which is why Drilbur sees more use than Sandshrew even though Sandshrew has Knock Off, and why Trapinch isn't among the A ranks despite its huge utility in specific match-ups. However, Mudbray's sheer stats may allow it to break this trend. Statistical bulk just under Spritzee's overall along with Stamina may let Mudbray act as one of the few hard checks to Aipom, along with giving it a great match-up against all sorts of physical attackers. Also, its Attack is really, really, high, although we'll probably end up with Trapinch-level damage (with slightly better coverage moves) since Mudbray will likely want to invest in its defenses. The amount of 1v1s Boxer should be able to win may be enough to propel it to the A ranks, although a lack of recovery will probably hinder it.
Predicted ranking: B+/B
Morellul
That typing is so good defensively, and it has good bulk and recovery to abuse its typing to its fullest. However, it doesn't look like Morellul does much else - Spore is nice, but then what? Morellul will probably spend more turns than not clicking its recovery move, and can't even do a whole lot with its free turns, which comes off as very Shellos-ish in how it would drain momentum. That said, its typing alone should grant it a niche on bulky teams.
Predicted ranking: B-
Stufful
Adorable Pokemon but as a Fighting-type, it has two major disadvantages in being weak to opposing Fighting-types and lacking Knock Off. It also doesn't bring anything major to the table, lacking U-turn or priority. However, its stats are quite good, having strong STAB moves and good bulk, and its ability could potentially let it 1v1 the Fighting-types it's weak to.
Predicted ranking: C+
Alolan Sandshrew
Ice is currently a pretty good anti-meta offensive type, which is why Amaura and Snover see any use. As such, Hail teams with Slush Rush Sandshrew could have some fun match-ups. That being said, there were points when Ground was a good anti-meta offensive type too, and that never got Sand Rush Sandshrew above C+. Also, the horrible weaknesses Ice/Steel brings will hinder any attempts at defensive Alolan Sandshrew. It doesn't have much competition as a hail abuser, but don't see Alolan Sandshrew being great.
Predicted ranking: C
Salandit
Frail and horribly weak to Diglett/Gothita - I would much rather use Gastly as my fast Poison-type wallbreaker. A vulnerability to Aqua Jet and faster Pokemon, as well as some difficulty in setting up, will likely mean that Nasty Plot variants are unlikely to actually sweep. Strong STAB moves and some utility in Knock Off and Will-O-Wisp are probably enough to make Salandit fairly threatening in a vacuum, but I don't think its strengths would warrant actually including on a team very often.
Predicted ranking: C
Wimpod
This one I'm not sure how to predict. Most of its stats are awful, and it does basically nothing on its own outside of fast Spikes (and I guess Scald burns). However, Wimp Out has a ton of outplay potential when paired with trappers, and can force some serious mind games.
Alolan Meowth
The change in typing removes normal Meowth's main niche as a Fake Out + Feint abuser, and the lowered Attack hinders a potential niche as a fast STAB Knock Off abuser. Nasty Plot could be somewhat threatening, but Meowth's difficulty in setting up, lack of damage before setup, not even being that strong after setup, and inability to deal with revenge-killers makes me very doubtful of its viability. I guess fast Parting Shot could be nice but I'm not sure if it's good enough to get Alolan Meowth to be more than a gimmick.
Predicted ranking: C-
Alolan Geodude
I think that Rock/Ground is more useful for SturdyJuice sets because it offers a Volt Switch immunity, so I would rather use Onix or regular Geodude there. Magnet Pull sets are most likely done better by bulky Magnemite because Magnemite resists Iron Head from Pawniard and Bullet Seed from Ferroseed. Galvanize means strong attacks but Geodude is sooo slow that I doubt it could reasonably attempt a sweep (even at +2 maxed out Speed, it's outsped by Jolly Choice Scarf Mienfoo). Its saving grace may be that it can do all three, giving it some level of unpredictability.
Predicted ranking: C-
Pikipek
Yes, it beats Tirtouga more easily than any other bird does with Bullet Seed, but its predictability means you can't quite call it a lure - Bullet Seed mostly just helps make it more difficult to wall. Not that you'll need to wall it if you're just running offense, because Pikipek has abysmal defenses and only decent Speed. I see little reason to use this over Doduo, Vullaby, or even Taillow - I'd say it's comparable to Mankey for type spam.
Predicted ranking: C-
Yungoos
A Scarf set could definitely force some fun mindgames with Stakeout and Pursuit as a pseudo-trapper, and we all know how good trappers can be. It's not very strong without Stakeout, though, certainly not enough to make up for its awful defenses. I doubt any other sets would be viable due to its low Speed.
Predicted ranking: C-
Alolan Rattata
STAB LO Hustle Sucker Punch and Quick Attack could be pretty annoying for some hyper offense teams, and it has the power to damage bulkier Pokemon with Return or Double-Edge as well. Of course, Sucker Punch's unreliability, only hitting 17 Speed, and absurdly low defenses will likely prevent Rattata from actually being any good.
Predicted ranking: D
Alolan Vulpix
Snover and Amaura are probably better.
Predicted ranking: D
Crabrawler
No Knock Off, no U-turn, no Poison Jab, and of course, no Regenerator - as a standalone Fighting-type, I'd rather use Mienfoo every time. Crabhammer is a bit more accurate than Stone Edge to hit Fire-types with but doesn't offer much else. Even as a Poison-slaying Fighting spam partner, Mankey is probably better most of the time, since on top of Earthquake, it also has Gunk Shot, Ice Punch, and U-turn.
Predicted ranking: D
Litten
Intimidate, U-turn, Fire typing and decent (though certainly not great) offensive stats should be enough to grant Litten a niche, however small. It probably won't be worth using because of its weakness to Stealth Rock, lack of coverage moves to threaten switch-ins with outside of U-turn (which it may not want to use too often because of its vulnerability to hazards), and the fact that some of the most threatening physical attackers are Water-type. I imagine it'll be around as good as Growlithe is now, though it would fit on a different team archetype.
Predicted ranking: D