Alright, after some playing around with some goofy, terrible HO teams, I think I'm about ready to give my first impressions. I'll start by first answering the questions.
1: What are you most excited to use in the new metagame?
Murder Dog. I know a lot of people think that Lycanroc-Dusk is simply outclassed by Terrakion due to its better bulk and ease of setting up, but I think that Lycanroc has some really great niches that make it stand out a lot more in comparison to the Sword of Justice. Most prominently, its slightly greater speed tier, new access to Play Rough/Psychic Fangs, priority STAB, and more fruitful coverage options. While Terrakion is definitely a more consistent breaker, I feel that Lycanroc-Dusk's main niche will come in the form of its coverage, which allows for it to annihilate Kommo-o--a Pokemon that's among the tier's more prominent defensive Rock setters
without set-up. Terrakion needs +2 to break through standard defensive Kommo-O, but Expert Belt Lycanroc needs nothing in order to achieve the OHKO.
252 Atk Expert Belt Tough Claws Lycanroc-Dusk Play Rough vs. 252 HP / 200+ Def Kommo-o: 365-432 (103.1 - 122%) -- guaranteed OHKO
+2 252 Atk Life Orb Terrakion Close Combat vs. 252 HP / 200+ Def Kommo-o: 325-383 (91.8 - 108.1%) -- 87.5% chance to OHKO after Stealth Rock
On top of this, its new access to Psychic Fangs lets it smash max defense Toxapex at +2 as well, a feat that it unfortunately was incapable of doing with Drill Run. Once more, here's another comparison!
+2 252 Atk Life Orb Tough Claws Lycanroc-Dusk Psychic Fangs vs. 252 HP / 252+ Def Toxapex: 312-369 (102.6 - 121.3%) -- guaranteed OHKO
+2 252 Atk Life Orb Terrakion Stone Edge vs. 252 HP / 252+ Def Toxapex: 226-266 (74.3 - 87.5%) -- guaranteed 2HKO after Black Sludge recovery
Though, I think the real kicker will come in the form of Screens HO. SD variants greatly appreciate Screen support to help it more safely set-up, and thanks to its Flying resistance, it is able to 1v1 Brave Bird Corviknight in the face of its Defog. Access to amazing coverage such as Drill Run, Psychic Fangs, and Fire Fang also give Lycanroc-Dusk a considerable offensive edge over Terrakion, and enables it to 1v1 things Terrakion otherwise struggles to best (most prominently Toxapex). Additionally, with its access to Tough Claws boosted STAB Accelerock and Close Combat, it can force switches against faster, frailer threats arguably even better than Terrakion, which can let it set up a Swords Dance, then wreak havoc. As such, I believe the standard item will likely be Life Orb or Expert Belt depending on what KOs are crucial for your set.
It's true that Terrakion overall is more consistent, but Lycanroc has some huge offensive advantages (namely its coverage and priority) that will shell it a good niche in the metagame, especially against the newfound Stall infestation.
2: What pre-dlc mons are expected to get hit the hardest?
Old habits die hard. These three I believe were hit the hardest, hugely because of what specifically came onto the scene.
Corviknight is now trappable and has some new competition in the form of Skarmory; the former flaw is more of note, because now it has to run a Shed Shell, which forces it to sack its passive recovery, or risk being trapped by Magnezone. It's true that Corviknight can just U-Turn out, but if it hopes to outspeed Magnezone, it needs to invest more speed at the cost of its valuable defense, or it needs to take a crucial hit before doing so.
Zeraora is hit hard with the return of Amoonguss and Tangrowth, both of which are incredible walls against its onslaught of coverage. I imagine that it will now need to run Fire coverage if it wants to break through these two, but even then, it will need Bulk Up if it wants to deal any actual damage. This is huge, namely because Bulk-Up Zeraora needs all 3 of its moveslots to maximize its effectiveness. If it gets rid of Knock Off, it can't beat Dragapult; if it gets rid of Close Combat/Drain Punch, it won't be able to as easily beat down the new Porygon2/Chansey, and Plasma Fists is out of the question. If anything, I would guess over its Fighting coverage, as Fire coverage still lets it beat Ferrothorn. Though, it now struggles a lot more offensively.
Chansey/Blissey. That is all that needs to be said about Kyurem. Before, it had little to no switch-ins, but its best checks just arrived, and they can Teleport right the hell out against it, forcing the Kyurem user to lose momentum. I wouldn't be surprised if Physical Kyurem becomes a thing just for these guys, but it loses a lot of its amazing immediate wallbreaking power in order to do so.
3: What pre-dlc mons are expected to benefit the most or see a rise in viability?
I can imagine Quagsire and Ditto once more rising in use now that they have actual stall cores to work with. Clefable beautifully supports them already, but now they have Chansey, Tangrowth, Slowbro, and Amoonguss to actually help support them. Ditto is a staple of Stall cores to prevent loose set-up, so I imagine that it's a huge winner as well.
Contrarily, I can also imagine some physical wallbreakers rising too to combat these Pokemon. Terrakion is the one that mostly comes to my mind, but Lycanroc-Dusk kind of gives it some competition due to its coverage. Terrakion is still amazing, though, and much less frail; its STAB Close Combat will be invaluable, especially.
The new DLC moves also bring some Pokemon further into viability: most notably, Indeedee and Rillaboom. The latter was already on the rise, but its new access to Grassy Glide gives its Banded set a greater role, and it appreciates Magnezone's support to beat down Corviknight and let it go to house. Expanding Force is fucking stupid if it's actually how it's coded in Showdown currently, so I can imagine that Male Indeedee would be a refreshed, interesting special wallbreaker in the tier consequently.
Flip Turn gives Keldeo more relevancy, too, as it can now pivot out against Toxapex and not fold momentum. Especially with Secret Sword being able to pry Chansey/Blissey apart, I can imagine that Keldeo will have new life breathed into it.
How do you feel about Galarian Slowbro and Urshifu?
It's cool! Shell Side Arm is a super cool tech with a really nice mechanic that I can imagine will be great against Stall cores, as Poison is a really great anti-meta offensive type, imo. I can imagine Specs variants can blow Tangrowth and Clefable away, and decently chunk things like Chansey, and Regenerator is an amazing ability for tank; its quad fighting resistance is neat, too. AV might be a sick tank, too. I don't think it'll be a top threat whatsoever, but I could see it have a niche as an anti-meta tank.
EDIT: Nevermind, Shell Side Arm sucks the way it's implemented. If it literally swaps to physical, then Galarian Slowbro kinda sucks. Shame; it has such a cool type combo.
I have no clue how to feel about it. It's most definitely not busted, but I find that it is actually a pretty neat stallbuster for its ability to penetrate protect and have critical hits land guaranteed on its moves, allowing it to break past its own attack drops and the opponent's defense rises. Being able to pop through Wish Clefable's protect is pretty big, as it can't just Wish willy-nilly on you, and having a Fighting-type along with Close Combat and Bulk Up is super useful too. It's got a pretty solid speed tier for the tier, too, but I can't help but feel that it is kind of underwhelming, honestly? At least the Rapid Strike version, anyway. It's cool, though, and has a niche, I imagine more as a balance buster.