https://pokepast.es/92fea695d14c0c5b
This is your team after my changes.
Your team is not bad at all but I have some concerns. Unlike what some people in the comments suggested, I'll keep Crobat for this team for the sake of originality - everyone can go build a completely prototyped team but where is the fun in that? Some of the most creative teams have made it to the top before, I personally did not imagine to see a Hail team peak 2000 for example at some point. Not saying that my changes will allow you to break into high ladder because this is mostly player-dependant moreso than team-dependant, but I wouldn't swap out a mon just because "it is not viable in OU". Yes, Corviknight is a better Defog user and Poison/Flying is not exactly good STAB to use on an attacker in a tier where it lacks the raw power to go past walls like Blissey, Slowbro, Ferrothorn or Toxapex and has no super-effective means of attacking them, but Crobat gets a quad resistance to Fighting, an immunity to Ground, and gets the ungodly combination of Super Fang and Taunt, which allows it to break defensive cores apart if they cannot pivot out to something that hits Crobat super effectively. It does have its merits that can be capitalized on in a creative OU team therefore, in my opinion.
Now for my concerns:
Something about Rillaboom, Tapu Lele and Psychic Seed Hawlucha doesn't sit quite right with me. This means to me that you have 2 terrain summoners but only one enables the potential of Hawlucha - if Lele is gone, you have 4.5 mons standing since Hawlucha is really only half a mon without activating Unburden.
3 Choice items users? This is absolutely not necessary in my opinion, especially that none of them has Trick or Switcheroo for example. Choice items are momentum sinks - you're locked in one move, and if it's the wrong move, your opponent can easily take advantage of that.
Therefore, I will effect the following changes in your team:
1. Rillaboom is fine
2. Hawlucha: change entirely to Kartana:
Kartana @ Choice Scarf
Ability: Beast Boost
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Leaf Blade
- Smart Strike
- Sacred Sword
- Knock Off
Kartana can abuse the Grassy Terrain set by Rillaboom. While both are grass-types and would share the same weaknesses, they're covered for by Heatran and Crobat pretty nicely for the most part. Rillaboom gets priority in the form of Grassy Glide, which would be your go-to move, whereas Kartana gets snowball potential after landing one KO. With a Scarf, it becomes reasonably fast, and while it is walled by specific threats such as Buzzwole, physically defensive Kommo-o or even Zapdos, this is why you have Nidoking, who handles those slower walls remarkably well. Kartana becomes your win-condition late game, and is less dependant than Hawlucha on the survivability of Tapu Lele to pull off its weight - and one more thing I dislike about Hawlucha is that without screens support, I find it too frail to set up comfortably in most games, Toxic-weak, and phazable with Whirlwind or Roar, all things that Kartana struggles to a lesser extent with.
3. Nidoking: change Scarf to Life Orb all-out attacker:
Nidoking @ Life Orb
Ability: Sheer Force
EVs: 252 SpA / 4 SpD / 252 Spe
Timid / Modest Nature
- Sludge Wave
- Earth Power
- Ice Beam
- Flamethrower
No Thunderbolt is not really an issue, you do not struggle with Water-types with an offensive Grass-type core. You do have Heatran, but you need Nidoking to be able to handle Ferrothorn, Scizor and Buzzwole with Flamethrower, and Zapdos, Kommo-o and Landorus-T and Garchomp with Ice Beam. If you really need Thunderbolt, you may feel free to swap Flamethrower out for it but I don't believe it is needed.
You may want choose to go Modest for the power - Zapdos outspeeding you is a non-issue, but if you prefer to hit 295 Speed, this is understandable. I don't think any one mon specifically threatens Nidoking enough and is in the 270-294 speed range to justify Timid personally, but then again, the extra power is really only noticeable in securing that your rolls will reliably KO threats like Toxapex, Ferrothorn... without prior chip or reliance on hazards. These moves are only walled by Slowking, Porygon-2, Chansey and Swampert, all of which are handled by your Grass core.
4. Crobat: change to speedy utility:
Crobat @ Heavy-Duty Boots
Ability: Infiltrator
EVs: 252 HP / 4 Def / 252 Spe
Jolly Nature
- Brave Bird / Super Fang
- Roost
- U-turn / Taunt
- Defog
I don't think Brave Bird is absolutely needed in this team. If you miss Flying-type coverage for whatever reason, go for it, but I really like Super Fang's additional utility. It might be iffy without Taunt, so you can slot in Taunt instead of U-turn maybe (bolded options should go together here) but if it's too risky / deadweight against offense teams, keep your set as is. Your EVs aren't particularly focused on any goal so I recommend going speedy support with max HP and Speed instead.
5. Heatran: fine, you may want to invest 120 in Speed to not let the (rare) Aegislash and Magnezone outspeed you, but you can keep it just as it is. No problems here - though I personally would prefer having Toxic somewhere, but this is personal preference.
6. Tapu Lele: At this point, I'd really like to recommend a Water-type to round the Fire/Water/Grass core, and preferably part-psychic to provide a Psychic and Fighting resist, which you may need after the additional changes effected to the team. Would you mind trying Slowbro? Radically different from Tapu Lele, but it provides slow pivoting with Teleport, and resistances that you can cover decently well. Since your team is weak to Knock Off at this point, allow me to introduce you to Colbur Berry Slowbro:
Slowbro @ Colbur Berry
Ability: Regenerator
EVs: 252 HP / 252 Def / 4 SpD
IVs: 0 Spe
Relaxed Nature
- Scald
- Slack Off
- Future Sight
- Teleport
You may not like it, it may be different from Tapu Lele but I believe it is worth testing out. Now by design, your team IS a little weak to Knock Off - maybe try Grassy Seed on Rillaboom to alleviate this weakness?
With all the effected changes, your team should be more balanced overall. Let me know what you like and what you don't like! We can always work out solutions. Good luck!