So I want to talk about Decidueye for a moment. Imo, it's going to drop down to RU unless Fighting-, Normal-, and Water-types are really damn common in UU (it loses to some due to their common sets / coverage anyway) and this thing has some really cool perks about it:
Decidueye @ Leftovers
Ability: Overgrow*
EVs: 248 HP / 8 Atk / 252 SpD (or 252 Def)
Careful (or Impish) Nature
- Defog
- Roost
- Spirit Shackle / Leaf Blade
- Baton Pass / Leaf Blade
Normally, you use Baton Pass in order to pass boosts, and while Decidueye can definitely do that with SD or NP (I'll get into these in a moment) Baton Pass also lets you bypass Pursuit, which is a pretty significant weakness it would otherwise have as a defensive Pokemon. Defog + Roost is absolutely amazing. We've seen what it does for other Pokemon, such as Gligar and Flygon, and Decidueye shouldn't be much different. Spirit Shackle is its strongest physical Ghost move and it takes a lot of counterplay away from your opponent once the opponent switches in (can't double due to the trap effect, so the target is forced to stay in unless it has U-turn, Volt Switch, or Baton Pass) Because the opponent can't double out to predict a switch, this takes a pretty significant tool away that most players use to take advantage of fleeing defensive Pokemon. Leaf Blade is an option over either Shackle or BP for very obvious reasons, but I honestly like the utility of both moves. EV spread and nature depends on which side of the spectrum you want Decidueye to tank hits from. Its typing works better for taking physical hits, but its stat spread works more for special hits (while still having Water, Grass, and Electric resists to back it up)
Decidueye @ Lum Berry / Decidium Z
Ability: Overgrow*
EVs: 252 Atk / 4 Def / 252 Spe
Jolly Nature
- Swords Dance
- Spirit Shackle
- Leaf Blade
- Baton Pass / Sucker Punch
You could probably run a bulkier EV spread if you wanted, but access to Swords Dance is really cool since it also gets Baton Pass. Spirit Shackle adds to this since it traps the opponent and keeps them from switching out as you may attempt to Baton Pass your boosts. Leaf Blade is needed coverage on fat Ground and Water mons and Sucker Punch is an option if you want to try to make Decidueye more of a standalone sweeper (I think it's better as a passer that can possibly sweep due to its average Speed) Lum Berry is probably the best item for such a set as it allows Decidueye to bypass status one time, but Decidium Z can give Spirit Shackle a major one-time power boost if you're wanting Decidueye to have a better shot at sweeping on its own.
Decidueye @ Leftovers
Ability: Overgrow
EVs: 248 HP / 8 SpA / 252 Spe
Timid Nature
- Nasty Plot
- Shadow Ball / Grass Knot / Energy Ball
- Roost
- Baton Pass
More of a dedicated Baton Pass set with Nasty Plot as Decidueye lacks the small quirks and counterplay measures that a physical set does with Spirit Shackle, thus making it harder to sweep. As this is more of a dedicated passing set, Decidueye has to pick which STAB move it wants to use. Shadow Ball has the best overall neutral coverage, but has a natural immunity in Normal-types. Grass Knot and Energy Ball are the best special Grass-type moves it gets (why no Giga Drain? :[) with the classic battle of consistency vs occasional higher power at work with that. Roost is mandatory to maintain health and allow Decidueye to possibly pass more than once in a game. Baton Pass is obvious as Decidueye is ultimately wanting to pass its boosts to a teammate. The EVs reflect Decidueye's more supportive role with max HP and Speed to take as many hits as possible while also passing as quickly as possible once it has the boosts. Obviously, you could mess with the EV spread some and add more power or bulk depending on what you want, but this is probably a good baseline to start with.
HIDDEN ABILITY
If / when Decidueye gets its hidden ability released, it's going to be an instant buff to physical sets. Long Reach will allow Decidueye to bypass all forms of contact damage (Rocky Helmet, Rough Skin, Iron Barbs, etc.) it would normally incur as well as allowing it to get around odd Static and Flame Body users (Aftermath too maybe?) This will make U-turn worth considering on the defensive set, although I still think Baton Pass would be better to dodge Pursuit.
FLAWS & OTHER NOTABLE MOVES
The typing in combination with its average Speed and Defense are its ultimate downfalls. The lack of coverage in its movepool certainly doesn't help any sweeping sets as it only really has Hidden Power for special sets and Brave Bird
(Acrobatics for the Z-move variants) for physical sets. That said, there are a couple of other support options it has access to. Haze is the most notable once since its typing works really well vs Virizion and it has good special bulk in general. Outside of that, Light Screen and Substitute are the only other noteworthy options Decidueye has available to it, neither of which I see it doing much with. Toxic is kind of bad since most Steel-types don't mind switching into Grass-types.
I think Decidueye will probably come down here to RU and end up doing pretty well. I doubt it will be meta-defining just because of the holes in its stat spread and movepool, but it should be very relevant (consistent top 20 mon, but probably no higher than just inside top 15 and possibly as low as top 35 if the meta isn't kind to it)