Since NatDexOU looks sadly lacking in any Gen VIII mons besides the obnoxious Corviknight that isn't either a starter with a busted hidden ability or a legendary (including Dragapult's busted self, which probably can't be made balanced without touching its stats), I'm going to try to improve two Gen VIII mons (of the ones not already submitted) enough that they might actually be viable with the changes. Fingers crossed:

Name: Copperajah
Type:
/
(Ground added)
Abilities: Sheer Force /
Water Absorb /
Heavy Metal (hidden ability; gains Water Absorb as a second non-hidden ability)
New Moves: Gyro Ball*, Knock Off*, Slack Off, Smack Down (Hidden Power and Toxic too as a Gen VIII mon.)
Removed Moves: [None]
Justification: Elephants need to drink
a lot of water per day, and the
Sword Pokédex's entry goes out of it way to say that Copperajah should at least be resistant to Water...in the umpteenth instance of gameplay-and-story segregation since every Water attack under the Sun and every other weather kicks its ass. I'm guessing the "resistance" bit was just a nod to copper apparently not rusting or corroding via water under most common circumstances, but I'm choosing to run with it because Copperajah has potential it falls more than a bit short of given its current stats, moves, and typing. I was initially going to make it Steel/
Water due to this but admitted to myself that I couldn't really justify the secondary Water type.
It's just as well though. For the most part and despite the unfortunate weaknesses Ground adds in some places, Steel/Ground is a better typing with Water Absorb as an ability anyway, letting this give quite a few Water types trouble for starters, e.g. basically walling everything Toxapex can do save for Knock Off, which Slack Off largely undoes, while threatening it back with STAB Earthquakes. Being Steel/Ground instead of Steel/Water also means it becomes the only mon in the game besides Shedinja that either resists or is outright immune to all three of the attacking pivoting moves. It also deals with at least two of the Tapus well enough, though Tapu Lele probably murders it even with just STABs. It's also a slow Steel that isn't instantly murdered by Magnezone, at least at full health, while now being immune to being bullied by its Volt Switch and Electric STABs. Water Absorb makes it the rare physical attacker that doesn't fear switching in on Scald. Finally, it offers different enough role compression that it should likely have some type of niche over both Excadrill and Hippowdon even if it's not as fast or strong as the former and isn't actually anywhere as tanky as the latter in general.
*It probably doesn't
need either of these, but I don't think either of these moves would make it broken per se, especially if it was somehow OU. So they "should" be fine. (Just like this thing's new secondary typing "should" arguably be Rock given its design rather than Ground [or Water], but it's already suffered enough
and Stakataka already exist, so we'll ignore that.)
Name: Charles Chaplin Mr. Rime
Type: /
(no change)
Abilities: Tangled Feet / Screen Cleaner /
Magic Guard (hidden ability; replaces Ice Body)
New Moves: [None] (Well, besides Hidden Power and Toxic as a Gen VIII mon.)
Removed Moves: [...None?*]
Justification: The primary justification for this is that of all the Gen VIII mons I could see getting the extremely powerful ability of Magic Guard, the mon that still deals with barriers, now with a carefree tap-dancing and "wintertime magic" twist, seems the most fitting. I mostly try to avoid giving out overpowered abilities where I can when improving things, especially since Mr. Rime isn't actually bad unlike, say, Stonjourner or Thievul. It's just outclassed despite its good movepool and relatively decent stats. So Magic Guard should easily push it over the edge into usability without any other changes and without outright obviating its other two abilities, though I do worry about how much of a push it may be. (See below.)
An additional inspiration for this was because it's bothered me for a while that the mon that's especially good at dancing gets an ability that seemingly implies it's clumsy in the form of Tangled Feet; it would be almost as jarring as this thing got Klutz to me, honestly. That said, I at least actually have seen at least a couple of people use Tangled Feet in passing in Doubles (those monsters) and can vaguely see the justification for it on Game Freak's part. So I ultimately decided that Ice Body gets the axe since Mr. Rime's six weaknesses and its resistances only to its own types aren't doing that ability any good anyway--same goes for its middling HP and Def.
*I wouldn't be surprised if this ended up needing to lose either Expanding Force, Freeze-Dry, or both with Magic Guard, if only because of Life Orb. I was tempted to suggest a revision of Magic Guard that dropped at least that interaction and allowed Life Orb recoil to damage you, but I'm not sure how easily that can be coded, so I'm not doing that (right now). Besides, I figure that since
Pokémon is technically a JRPG, I might as well let Mr. Rime potentially amass enough power to kill
God Arceus...at least until the Golden update nerfs it in some way for breaking the game.