Crobat with Gunk Shot: Improves its Band and other offensive sets as a harder hit on Fairies that doesn't cause recoil in exchange for questionable accuracy. Big thing with this though is that it doesn't solve any of Crobat's problems with Rock- or Steel-types that become even more relevant with Mega Steelix and Gunk Shot in general is not a fun move to be locked into.
Fluffy Arcanine: This could be an interesting boost to Arcanine, seeing as the Fire weakness is essentially a neutrality thanks to its own natural resistance. Arcanine still suffers from its hazard weakness and from Morning Sun being an unreliable recovery method due to its low PP and other weather cutting into its day in the sun. However, it does survive some neutral attacks it would not like taking previously:
252 Atk Cobalion Close Combat vs. 248 HP / 252+ Def Fluffy Arcanine: 64-76 (16.7 - 19.8%) -- possible 7HKO after Leftovers recovery
+2 252 Atk Cobalion Close Combat vs. 248 HP / 252+ Def Fluffy Arcanine: 127-151 (33.1 - 39.4%) -- 15.1% chance to 3HKO after Leftovers recovery
252 Atk Terrakion Close Combat vs. 248 HP / 252+ Def Fluffy Arcanine: 82-97 (21.4 - 25.3%) -- possible 5HKO after Leftovers recovery (Stone edge will kill after this obviously but its a nice testament to Arc's newfound physical bulk)
+2 252 Atk Terrakion Close Combat vs. 248 HP / 252+ Def Fluffy Arcanine: 162-192 (42.2 - 50.1%) -- guaranteed 3HKO after Leftovers recovery
192+ Atk Buzzwole Focus Punch vs. 248 HP / 252+ Def Fluffy Arcanine: 114-135 (29.7 - 35.2%) -- guaranteed 4HKO after Leftovers recovery (used the calc set for this)
252 Atk Life Orb Infernape Flare Blitz vs. 248 HP / 252+ Def Arcanine: 90-107 (23.4 - 27.9%) -- possible 5HKO after Leftovers recovery (remember that Arc no longer resists Fire)
+2 252 Atk Life Orb Infernape Close Combat vs. 248 HP / 252+ Def Fluffy Arcanine: 181-214 (47.2 - 55.8%) -- 24.2% chance to 2HKO after Leftovers recovery
Of course, with Arcanine losing its resistance you'll have to watch out for things like this:
252 SpA Chandelure Fire Blast vs. 248 HP / 0 SpD Fluffy Arcanine: 234-276 (61 - 72%) -- guaranteed 2HKO after Leftovers recovery
Arcanine becomes a better check to the physical attackers in the tier. However, the common use of non-contact attacks like Stone Edge and EQ (both of which it is weak to) and its susceptibility to hazards means that it probably wouldn't go above B/B+ optimistically speaking.
Empoleon with Roost: Let me preface this by saying that emperor bird deserves this way more than fucking Mantine (seriously, have you seen a ray roost?). This should improve Empoleon's walling capabilities significantly and make it more of an option to consider over other hazard removers like Tentacruel. In particular its matchup against Latias that don't carry Gigavolt Havoc will definitely improve as Latias can't 2hko Empoleon with any of its moves before a boost and Empoleon can Toxic/Roar it out. However, the option of carrying both SR and Defog on Empoleon becomes a little harder to justify as without Toxic or Roar it can become set up fodder for the likes of Raikou. That being said, this will definitely improve Empoleon's viability as a defensive threat
Mismagius with Ghost/Electric typing and Volt Switch: With the Rotom treatment, Mismagius adds resistances to Electric and Steel at the cost of still getting fucked by Krookodile. This actually makes it an OK check to Scizor instead of getting boned by it like before:
252+ Atk Life Orb Technician Scizor Bullet Punch vs. 0 HP / 4 Def Mismagius: 105-125 (40.2 - 47.8%) -- guaranteed 3HKO
Missy can wisp it to make it useless or just straight up kill it with Mystical Fire if it opts to run that.
(note that if Offense SD Scizor grabs a boost Missy is dead meat)
Outside of this, I don't know how much viability this will actually have because even with a new STAB Mismagius is still not super powerful without a Nasty Plot boost and Krookodile is still everywhere. I guess it has a niche in being a spinblocker who can beat Mega Blastoise 1v1, although it needs to carry a Life Orb or Specs to guarantee the KO.
EDIT TO MISSY: Also checks MPidge decently I guess
252 SpA Pidgeot-Mega Hurricane vs. 0 HP / 0 SpD Mismagius: 89-105 (34 - 40.2%) -- guaranteed 3HKO
252 SpA Pidgeot-Mega Heat Wave vs. 0 HP / 0 SpD Mismagius: 102-121 (39 - 46.3%) -- guaranteed 3HKO
252 SpA Mismagius Thunderbolt vs. 0 HP / 0 SpD Pidgeot-Mega: 306-362 (99.6 - 117.9%) -- 93.8% chance to OHKO
Crobat with Gunk Shot: pretty shaby as buff. A strong Poison move is nice to have, but Crobat has problems to get past its checks i.e. Steels and Rocks despite Super Fang. It feels not enough for me to make Crobat really better; maybe with a better ability is enough to make this bat stronger.
Fluffy Arcanine: Nice buff flavour-wise, but the lack of reliable recovery, weaknesses to all hazards and bad defensive typing (even with Flaffy boost) really condemn this pokemon. Flaffy doesn't work towards physical not-contact moves i.e. Earthquake, which is highly spammable with that high base power and accuracy. Even in this case two buffs seems adequate to me, because Fluffy and Intimidate are in practice equivalent considering all the drawbacks.
Empoleon with Roost: very good nomination because now we have a bulky Steel with reliable recovery with a good defensive typing and Scald at its disposal. It has a huge 4MMS because it can't run SR, Defog AND Roost all in one set: uninvested Scald is not impressive at all and Roar is nice to avoid being set-up bait (but now you die to Taunt). Probably, you will choose between SR and Defog to run Roost. Still, reliable recovery is a great addition to a viable Steel type and this is a solid submission.
Mismagius with Ghost/Electric typing and Volt Switch: A buffed Rotom, but I feel that this theorymon is a little poor design-wise. Good monster, but almost it copied&pasted regular Rotom. It has nice support moves and Nasty Plot to be an effective cleaner, but in this case Volt Switch neuters your SpA boosts.