With DLC (more or less) right around the corner, I'm going to go over some stuff coming back in both DLCs as well as speculation of the new stuff. I can speculate on what is coming back but for the sake of this post I will only be going over
things that are confirmed or stuff could be reasonably assumed such as starters returning. Obviously spoilers if you wish to avoid the info.
Let us start off w/ basically the only mon that matters, rest are mediocre to awful in comparison

This is my mans 123 attack Reckless STAB Flare Blitz. Emboar has two things that set it apart from its Fire-type competition. One, really good coverage. Wild Charge, Head Smash, Sucker Punch, Flame Charge, and special options that aren't entirely awful considering Emboar has 100 spa in Grass Knot and Scald. Second, STAB Close Combat (or Superpower assuming SS somehow doesn't give Emboar CC) which is nice for Chansey albeit Fire STAB is usually more useful for most MUs. This thing has a lot of potential, don't sleep.
While I'm here might as well talk about all of the notable returning starters, I'll cover it in FWG order and make one section for all the ones w/ likely small niches.

This will be good for sure. 80 initially falls just short of making it busted but Speed Boost is an incredibly good abilty and on a mon w/ incredible bases and a good movepool Blaziken will be good for sure. This is a weird case as you'd usually point to how this mon did in previous gens but Mega existed in SM and ORAS so it'll be interesting to how Blaziken fares w/o 160 attack. It's movepool is clean and it has two high BP STABs so there is no way this thing will be not good but I can't see it being defining.

Blaze go brrrrr. I was hyped about this thing but all it has over Cinderace is boosting, Fighting STAB, and Blast Burn. To be fair, those all are pretty good traits but in a tier that will be saturated w/ excellent Fire-types this thing will struggle to stand out. Not bad by any means it may take over Cinderace's role if being special proves to be more useful than being faster and stronger physically.
also keep in mind 100% superior emboar exists
Can't see these two being particularly useful. Movepools are not nearly useful enough compared to their counterparts and overall their abilities and stats aren't conducive to a good niche. Delphox maybe can do smth w/ CM idk I really doubt it. Eruption is also not impressive when all of Typloshion's competition gets Blast Burn.

Gets Yawn so maybe this'll be good? Idk losing out on Mega's bulk and strength is pretty huge and while it frees up the item spot what are you gonna put there considering Custap doesn't exist. It's a bulky water w/ no boosting and no reliable recovery and it's stats are underwhelming. Thought I'd mention it just to reminisce about Mega Pert.

No Custap is a huge L but Empoleon's typing is clean. It's quite situational but sometimes teams are just unable to break Steel/Water w/o dying back. Torrent boosted Hydro Cannon can't be bad.

Wuh oh. While losing out on Hidden Power is pretty big Protean is just a ridiculously good ability backed up by solid stats and an insane movepool. Gunk Shot especially seems useful considering the dominance fairy types have rn. Gren is Gren it'll be really good, not much to say.

They lack a good secondary typing as well as mediocre speed tier for offensive mons w/ no bulk. Dead on arrival.

The return of the king. Contrary+Leaf storm is a match made in heaven in addition to being able to Sub Seed Stall other shit. Until its fren Regi is back tho this thing will struggle to stand out, I mean you could just use another Steel but that's lame. Barring Prim, Serp does not really have useful great matchups versus the best mons and w/ better Fire types returning and Dragons still poised to rule Serp will have its fair share of issue. Not bad by any means but this thing is hard carried by Contrary and when Contrary isn't useful it really shows Serp's weaknesses.

Grass is not a good typing I doubt any of these mons have any reason to use them over Whimsi, Rillaboom, or Serp. Torterra may be cool when Custap returns and Sceptile is fast so maybe those two can find a niche but I really doubt the other two have any chance.
Time to move onto returning mons that are confirmed for the first DLC.

130 special attack is really good, Sturdy is really good, typing is useful in SS. This thing is poised to be one of the better things in the tier and it stands as one of the Sturdy mons that can beat stall. Metal Sound+Electro web looks very clean maybe Specs could work. Rotom-H, Zera, Hax are all posed to be nuisances but Magnezone is for sure an excellent addition to the tier, finally a great offensive Steel.

A Haban Dragon w/ 102 speed seems very clean and Scarf has potential for beating all the Scarfers around 95. You also have options like SD and Fire Blast for dispatching certain stall threats and two high BP stabs are very nice. Garchomp will find quite a nice niche in SS especially before DLC 2.
Fat motherfucker now look who's in trouble. This gen Chansey can finally run both Charm+Stoss which means Charm won't have to rely on random coverage backed up by no special attack. The new face in Darm-G may pose an issue but nonetheless Chansey will be a defining stall pokemon. Blissey may be worthless for running Kee or some shit I really doubt it unless Mail becomes a thing in which case that'll make Blissey a menace.

The typing is great both defensively and offensively and Metagross packs a punch. Also gets solid moves like BP, EQ, T Punch, and Hammer Arm which help give Metagross options although most of the time you will be clicking Meteor Mash. Berries such as Occa and Shuca look clean as hell allowing Metagross to weakness Sturdy mons in range of BP as well as picking up some MUs from the Berry. Also 70 speed is quite salvageable I can see this being quite good before DLC 2, RegiSerp and Zapdos will pose an issue for it and while not a dealbreaker it will def hurt Metagross's viability.

Oblivious is such a great ability for a stall mon albeit the loss of Battle Armor is very sad. This will fulfill the role of a stall mon that can take on Corv and Mandi. The choice between Amnesia and CM will be a rough one considering both have sizeable trade-offs. The loss of essentially 70 defense, being Trickable now, on top of loss of Battle Armor is pretty huge but it can't be awful considering it's a stall mon w/ Oblivious.

Despite having seemingly excellent MUs last-gen this thing never really came to be used. While its movepool, typing, and abilty all seem great on paper the stats really hold it back. Serviceable bulk doesn't really cut it and 85 speed is quite mediocre. This thing always gets a lot of hype for beating the best shit but then you look at its abysmal MUs w/ other mons.

This mons finally gets Charm+Amensia+Sap Sipper. A water that beats grasses is brazy but honestly other than that I would never use it over other bulky waters. Huge Power may alright but I'm not a huge fan of mons that are this slow and 436 attack is not impressive as what it first seems.

This thing has seen quite a fair amount of hype and I think for good reason. 135 spa 100 speed are quite good w/ two offensive STABS but there are few things holding it back from stardom. 85/65/105 bulk while serviceable is not going to be tanking any decently strong hit. Rock and Water are fairly common typings that hurt Volc. Regardless, this thing is fast Centiskorch that can beat a lot of the notable stall mons and beating Sylv is cool. I can see this being quite good but it's going to have 4mss which is always a pain.

Fair warning, this one hasn't been confirmed but I heard it was coming back once forget from where. Adaptability is nutty ability especially when the move you're boosting is Hyper Beam. Anything that is not a resist is dead on the spot and PZ has potential to cheese resists w/ NP. While the loss of Z-Conversion is a bummer PZ will remain an incredible offensive presence in 1v1.

Download is a brazy ability I can see this mon finding a niche BoltBeam good bulk is noteworthy. Best physical boosting option being Reflect stings but I'm sure P2 will find a niche based off its sheer bulk.
Time to talk about legendaries returning in the second DLC.

Observation: This is a great opportunity for slaughter. 80/150/150 bulk is crazy especially considering the excellent defensive typing that is pure Steel. Registeel also has ID, Amnesia, and Counter helping it supplement its already excellent bulk. Regi is most infamous for RegiSerp which is an insanely good stall core. Regi will be one of the most important parts of the metagame and may finally overthrow the reign of Prim and Sylv.

Already a really good Pressure stall mon w/ potential for offensive sets and the addition of Eerie Impulse is really nice. Such a strong mon w/ a lot of versatility Zapdos will be an incredible pick although losing Hidden Power is a pretty huge hit for Specs, Hurricane may patch that up although. Zapdos is set to be one of the most defining mons in the tier.

Gets Stored Power and Cosmic Power now, wuh oh. Cress is already difficult enough to kill and it finally got psychical boosting which is a pain. Dark mons can't even properly hold Cress back because of Moonblast breaking Cress is insanely difficult you're going to have to rely on your Trick mon being able to kill Cress before it kills you, which is easier said than done. Darm-G can't properly dent this thing which is really scary the general de-power creep of SS makes Cress look nutty. Not unbeatable, however it does cover a fair amount of attackers in the tier.

Worse Zapdos unless you're lucky. Fr tho, it does have Taunt over Zapdos so it will be Zapdos that can beat stall mons but not being able to wall things. It has a useful enough movepool it'll get by.

SS may finally get a good ground, although Chomp will fill this role before DLC 2. Intimidate and Lando's excellent stats are also nice albeit Lando will not see as much success in 1v1 as it does in OU. Speed tier and abundance of bulky waters is always an issue but it's bound to be a solid breaker.

While it lost being an insane nuke w/ the loss of Z-Moves Koko is still really strong in addition to an annoying stall set that prone to become even more annoying w/ the introduction of Eerie Impulse. Band can also break dragons now w/ Play Rough. If it was banworthy last gen I can't really see that changing this gen being both a fast nuke and stall mon is incredibly annoying to handle. A suspect is probably inevitable and I think that's reasonable considering the insane potential Koko has.

Lost Z-Reflect but it's still strong as all hell so it'll be fine. Losing the ability to be a physical wall is a bummer but it's still ridiculously strong and it's already been shown that strong fairies are very good. Also w/ option like Taunt and Nature's Madness make Lele look really good going into SS.

Also got Play Rough allowing to break dragons this will be incredibly strong albeit awkward typing and speed tier hold it back. It's strong as all hell and also somewhat bulky so it'll be quite solid but it's hurt by its typing more than it's helped by it.

No Tapium-Z really hurts Fini. Typing is nutty, bulk is good, speed tier is solid, it gets Taunt, and it can't be statused so it's bound to be usable but it's going to struggle to break things. It'll be a stall check taking over Corv's and Mandi's job assuming something else hasn't already stepped in.
Time to speculate about the new mons, seeing as some of them don't really have info to really talk about I'll refrain from those. That means no Galar formes.
Typing seems really bad. Unless it has some insane STAB backed up by excellent offenses it'll fall by the wayside. Maybe some Taunt variant could be cool but a lot of things are going to half to go right for this thing to be good.
Assuming both have Regi-like stats they'll be very good. Both are really nice defensive typings but there is also the possibility these go the offensive route which would be interesting. Electric seems solid albeit Lando-T and Chomp will be scary. My issue w/ Dragon is that it'll most likely end up being a Slow Haban Dragon which is always bad. Doesn't help w/ the abundance of fairies that currently rule the tier. To me it seems these two will be hard to justify over Registeel in most cases, doesn't mean they'll be bad or outclassed just they'll be more nichepicks.
Keldeo and Pangoro typings both seem like Fairy food. I'm unimpressed by Keldeo and unless this one is significantly stronger I really doubt it'll be able to hold water(heh). The Pangoro one especially is just going to be unable to touch fairies and die back. They'll find some niche for sure if they're strong enough but I'm not holding my breath.
In conculsion
While there are still a few things to go over such as new moves for returning mons and how viable old mons will be going into DLC but this post is already taken long enough I may make a follow up to cover those fronts. One thing I would like to note however is that a good portion of the returning mons seem like Darm-G food. DLC seems significantly more interesting than the SS we currently play and I can't wait to see how the new faces fare in the meta.