Just a few nominations :)
Electric
A->B - This new metagame has not been kind with Magnezone. Even though it's still a great Pokemon on paper, it struggles to find a notable niche on Electric Teams. And here's why
1) Magnezone's main selling point back in ORAS was arguably its great Steel typing, that helped Electric gain notable unique resistances to Normal, Psychic, Dragon, Fairy, Rock and Poison. Now, with the introduction of new mons like Tapu Koko (with Dragon immunity), Alola-Golem (with Poison and Normal resists) and Alola-Raichu (with Psychic resist), Magnezone's plethora of unique resists isnt as big as it used to be, leaving only Rock and Fairy as notable ones that can't be filled by other (viable) teammates.
2) Electric's new more offensivelly inclined archetype and the introduction of new special attackers like Xurkitree, Tapu Koko and Alola-Raichu leave Magnezone in a tough spot for a team slot, especially if you look at its underwhelming Speed stat and its lack of coverage outside of Hidden Power to hit Electric immune mons, with its moveset being extremely predictable.
3) Magnezone used to run, in ORAS, 2 common types of sets. One was a revenge killer Scarf set, with HP Ice, to pick on Landorus, Garchomp and other threats like Dragonite or a Specs set with HP Grass, trying to lure and dispose of the Water/Grounds and other Electric immunities so teammates like Mega-Manectric or Thundurus could sweep. Looking at the SUMO metagame, both of those sets are extremely outclassed by the new additions like Xurkitree and Alola-Raichu, both sporting much better coverage and being more unpredictable than Magnezone, while having a much better speed tier.
4) Flash Cannon is not as great as it used to be in ORAS. With the introduction of Electric Surge and the new mons with Fairy coverage (like Xurkitree and Tapu Koko), its ability to hit Fairy types (especially Calm Mind Clefable, which used to be a pain for Electric monos), Rock types (like Cradily) and most Dragon types neutrally is not as beneficial as it used to be, considering Electric teams have now better ways to break those types with either coverage or insanely powered Electric STABs
Don't get me wrong though, I still think Magnezone is a great mon, but it's extremely hard to fit on most Electric builds, qualifying it for B rank
Since we're at it, I'd suggest everyone try Magnet Pull Magnezone on Electric. It's imo the best set this gen (where it still only has a notable niche), disposing of threats like Ferrothorn and Excadrill (locked in Rock Slide), making sweeping types like Steel extremely easier for mons like Xurkitree, while also being able to remove Ground's main wincon.
Conclusion: Despite still being a great pokemon for electric monos, Magnezone struggles to fit in the more offensive Electric teams, due to it's poor speed stat and extremely predictable movepool. B rank
B -> C - Raikou's B rank is imo completely undeserved, because this mon really struggles to find a real niche in any Electric team, especially after the introduction of Alola-Raichu. Raikou's main uses in ORAS were its good coverage moves in Extrasensory and Aura Sphere, adding some diversity to the table of Electric teams' bland coverage moves, easing some matchups like Poison, Grass and Rock (hitting notable troublesome mons like Mega Venusaur, Ferrothorn, Cradily and so on...), and its above average natural bulk, while sporting some good offenses.
Thing is, with the introduction of Alola-Raichu, most of those threats can already be taken care of with the newcomer's great movepool, including Psychic, Focus Blast, Surf and Grass Knot. Raichu also has an amazing ability in Surge Surfer, being able to speed control, and a good Psychic typing, giving it a resistance to Fighting and Psychic moves, while giving Psychic a much appreciated STAB bonus, helping break defensive threats like Mega Venusaur much easier.
So, what's left for Raikou? A sub-calm mind set is a good option, but it's extremely hard to fit on Electric's new builds, when the type relies on offensive pressure and a lot of switching/pivoting to gain momentum.
Conclusion: Raikou is a very niche Pokemon that is outclassed and extremely hard to fit in monotype SUMO. C rank
Fairy
S -> A - I already brought this before, but I think Clefable's rank is still reminescent from its ORAS roles in Fairy teams, although with the increase of Babiri Clefable's usage, it gained another solid niche that I did not see before. Even then, it's still not enough for a S ranked mon imo. I'll just quote my other post:
With the introduction of all new fairies and the harsh team slot competition, Clefable isn't as good as it used to be in Fairy monos. It's definitely not as vital for the type as other S-ranks like Klefki, Azumarill, Koko and Magearna, especially because of its single typing that dont benefit the type with more resistances. Don't get me wrong though, Clefable is still great, it just faces fierce competition from the likes of Magearna, Tapu Fini and Koko and other fairies as a Calm Mind (or just setup in general) sweeper. It also faces competition from Tapu Fini as a status absorber. It only has a few niches right now from what I've seen, setting up Stealth Rock and being a bulky CM sweeper.