SM OU Any ways I can improve this team I made?..... Rate this team................

[Aqua987]

Venusaur @ Venusaurite
Ability: Overgrow
EVs: 252 HP / 252 SpA / 4 SpD
Modest Nature
IVs: 0 Atk
- Sludge Bomb
- Giga Drain
- Synthesis
- Hidden Power [Fire]

This is the overall backbone of the fire, water, grass core this team offers. Its the sponge of the team with thick fat, but can still dish out some good damage with Sludge Bomb. It has hidden power fire for those pesky Ferrothorns or Scizors. Synthesis is just to heal, just like giga drain is. This overall deals with Tapus that are quite common, and almost always tanks a choice band brave bird from a tapu koko.
252+ Atk Choice Band Tapu Koko Brave Bird vs. 252 HP / 0 Def Venusaur-Mega: 330-390 (90.6 - 107.1%) -- 43.8% chance to OHKO,
and then revenge kills it with sludge bomb............

Gyarados @ Flyinium Z
Ability: Moxie
EVs: 252 Atk / 4 SpD / 252 Spe
Adamant/Jolly Nature
- Dragon Dance
- Bounce
- Waterfall
- Earthquake

The sweeper part of the fire, water grass core. Heatran makes perfect bait for earthquakes, and that allows this to set up a dragon dance almost free, and under A-Ninetails Auroa veil can be quite troublesome for any given enemy. Z move moxie boost to get more damage off overall, and the speed you'll be getting from dragon dance to factor in..........

Heatran @ Air Balloon
Ability: Flash Fire
EVs: 252 HP / 252 SpA / 4 SpD
Modest Nature
IVs: 0 Atk
- Lava Plume
- Stealth Rock
- Roar
- Earth Power

The main hazord setter and special defencive wall, air balloon is to be able to switch in once on a given earthquake, or to wall out opposing heatrans. Roar is to switch them out if they try to set up. Earth power is to deal with opposing heatrans. Under Aurora veil from A-Ninetails its an overall bulky wall, that can lave plume and inflict massive damage and deal massive hazord damage.........

Ninetales-Alola @ Light Clay
Ability: Snow Warning
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
IVs: 0 Atk
- Aurora Veil
- Blizzard
- Moonblast
- Freeze-Dry

The guardian angel of the team which can aurora veil to support this team overall, and can dish out massive damage to any given enemy, and deals with gyrados with freeze dry or pokemon that would otherwise cause a problem. Light Clay is to maximize the time that veil is up.......

Skarmory @ Leftovers
Ability: Sturdy
EVs: 252 HP / 232 Def / 24 Spe
Impish Nature
- Defog
- Roost
- Brave Bird
- Whirlwind / roar

The wall and pest to sweepers. To be able to be a physical wall that destroys those teams that are dependant on hazards and can take Roost to heal. Defog is the hazord removal, and brave bird is to hit fighting types or grass types. roar or whirlwind to switch the pokemon they have out........

Garchomp @ Choice Scarf
Ability: Rough Skin
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Earthquake
- Stone Edge
- Outrage
- Iron Head

Classic scarf chomp without fire blast because I feel this team has enough coverage for threats that need a fire hit. Iron head is to hit fairys, Stone Edge to hit flying types, Outrage to hit hard, and earthquake to be annoying to any given opponent.......

 

[Stories]

Alolan Ninetails does not fit with the defensive nature of your team; Aurora Veil is best used by somewhat bulky set-up sweepers, such as Magearna and Zygarde, as opposed to defensive Pokemon like Mega-Venusaur and Skarmory. I'm going to scrap this slot and come back to it later.

Mega Venusaur is best ran defensively, because even without a lot of offensive investment, it can still hit very hard, boasting a 122 base Special Attack stat and respectable 100 base Attack stat. Because you have a Heatran and and Flyinium Gyarados, Hidden Power Fire isn't really needed; I'd replace it with Earthquake, as that threatens Magearna, Heatran, and Magnezone, which are more troublesome to your team than the likes of Ferrothorn and Mega Scizor.

Venusaur @ Venusaurite
Ability: Chlorophyll
EVs: 248 HP / 88 Def / 156 SpD / 16 Spe
Calm Nature
- Giga Drain
- Leech Seed
- Synthesis
- Earthquake

Gyarados is a decent wallbreaker, but I would give it a Jolly Nature so it can outspeed Pokemon like Tapu Koko after a Dragon Dance, which it can't do with an Adamant Nature. I would also make Gyarados have Intimidate, which can help it set up versus Physical Attackers like Tapu Bulu, Mega Lopunny, and Kartana. aside from that, I think it's fine. Taunt or Substitute could be used over Waterfall or Earthquake, but those moves provide nice coverage so it's really up to you.

Heatran is a fine choice here, but I'd change the set and spread to better suit the meta. Change Lava Plume to Magma Storm, which traps opposing Pokemon and basically just does a lot of damage. I'd also change Roar to Stealth Rocks and give Heatran max Speed and HP, which lets it deal with Pokemon like Tapu Bulu and such.

Skarmory is really just not that good right now... If you want a Defog user with good physical bulk, Mew would help with your slight Mega Medicham and Hawlucha weakness.

Mew @ Leftovers
Ability: Synchronize
EVs: 240 HP / 156 Def / 112 Spe
Timid Nature
- Ice Beam
- Will-O-Wisp
- Soft-Boiled
- Defog

Garchomp is a fine scarfer, especially considering your potential weakness to Volcarona, but Iron Head is not needed as Earthquake does just as much to Fairy types like Tapu Lele, and does more to Fairy types like Tapu Koko and Mega Diancie. Iron Head can be replaced with Dragon Claw; which is a good STAB move that doesn't involve being locked into a move, or Toxic; which can be useful against common Garchomp switch-ins like Lando-T, Tangrowth, and Mew.

As for the Ninetails replacement, I think Tapu Koko can close the team up well. It provides a lot of offensive coverage and momentum and can clean well late game. Tapu Koko can also deal with bulky Water types that could potentially wall out your team. To beat Water types like Toxapex better; you could Substitute Taunt with Roost for longevity .

Tapu Koko @ Zap Plate
Ability: Electric Surge
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
- Thunderbolt
- U-turn
- Hidden Power Ice
- Taunt