Okay, so there is definitely a fair amount of Pokemon that ought to be taken into considering in light of the changes for USUM. I don't think much has objectively dropped in viability (barring some things that are just now outclassed like Lumineon mentioned above), but there are certain things that can be discussed for moving up at least a sub-rank or more:
Altaria
Defog is one of Altaria's most important new additions to the metagame, as it allows the Pokemon to finally carve itself a niche among other defensive Pokemon. Despite Altaria's weakness to Stealth Rock, Altaria's immunity to Spikes, great bulk, and access to Natural Cure helps alleviate pressure within its role of a support Pokemon, and Roost helps give it leeway to stick around and stay healthy throughout the battle. Natural Cure, in particular, is super neat for Altaria as that allows it to function as a status absorber as well as basically 1v1 a lot of defensive Pokemon like Gastrodon and such that also likes to carry Toxic. All of these traits allow it to match up well against a lot of hazard users such as all of Defensive Mesprit, Ferroseed (thanks to Flamethrower), Weezing, Miltank, Pallossand, and various others. It's also one of the most durable choices in a tier, with the ability to fit comfortably well in a lot of balance and stall builds and do its job just fine. In terms of possible sets, I've used both physically and specially defensive variants and they both have plenty of their perks; the former functioning as a very solid switch-in to cb/scarf primeape and passimian and still being capable of solidly checking stuff like Eelektross and Magmortar (two Pokemon that are good at breaking balance) by virtue of its typing and naturally high special defensive stat alone and stalling out Regirock with Toxic+Roost a lot more reliably (and thus being better at keeping hazards away vs it), while the latter just being a super solid special sponge against a lot of other stuff like Pyroar more reliably. I wouldn't quote me on any other possible ways of using this but like an offensive Defog set with Draco / Defog / Fire Blast / Roost doesn't sound that bad either, since it's basically a pivot to a lot of status and Magmortar an Eelektross without being too passive and still supporting the team. All in all, Altaria has a new tool and does a fantastic job of utilizing it, and definitely warrants a rise in at least
B rank.
/
Rotom-Frost and
Fan are both particularly effective new users of Defog due to their unique pivoting abilities for a defogger and their respective offensive and defensive presences. Frost, in particular, is a Pokemon I feel has some pretty untapped potential, and I've retained that stance ever since the rising of Piloswine. With Defog, Rotom-Frost can pressure Qwilfish, Palossand, and Mudsdale, and Subzero Slammer with Electric coverage can dent a lot of teams lacking Lanturn in general. Defog + Blizzard is admittedly enticing as well!
B- rank seems like a fine jump for Frostom.
As for Rotom-Fan, it's more effective defensively due to its Flying typing that lets it check stuff like Gurdurr and switch into Fighting and Grass types well, as well as soft check Pinsir with its bulk with Physically Defensive sets. It can use Will-o-Wisp nicely to cripple Golurk, Regirock, Golem, and Mudsdale, and although it lacks Roost for longevity, it can reasonably fit options like Pain Split or even Sitrus/Iapapa Berry to somewhat mitigate that. I think a rise to
C-rank is reasonable for it.
This Pokemon has plenty of room for exploration in general, but I feel that
Silvally-Water and
Silvally-Steel are Pokemon that I feel have the most notable improvement from Defog, as they respectfully are solid at taking on the majority of hazard users with the coverage they have and have varying degrees of defensive and offensive utility on top of Parting Shot and Tailwind. Their main flaw is lack of recovery (though Steelvally somewhat mitigates this through its resistance to Stealth Rock and immunity to Toxic Spikes/Toxic) which is limiting for their jobs, but they do it well enough to where Silvally-Water can move from UR to about
C and Steevally could stand to rise to at least
C as well.
Servine actually competes with Lurantis well as a defogger that has options like Taunt and Glare to separate itself, combined with slightly better Defense with Eviolite to more actively switch into stuff like Regirock. It is also notably faster than Lurantis, all while sharing the trait of pressuring a lot of Stealth Rockers and beats Ferroseed pretty well. Mirror Coat is a niche but usable option that can be used to lure Fire-types as it can take a hit and KO back, and can also let it come in on Weezing and deter it from attacking. UR to
C is fine in my opinion.
I want to cover some others in a less Defog-centric post another time, but I think Pokemon like Palossand and Togedemaru (both of which I feel can rise a subrank due to improved utility options that lets them function more effectively and especially in Toge's case, with Scarf being recognized as a set with a good niche that outspeeds base 95 Scarf mons, grabbing momentum, and being good at threatening common offensive Pokemon like Swanna and Archeops) are good discussion points to be had for the time being.
Lastly, I leave my man
GenZeon in charge of bringing up Bellossom to be ranked again.