Guts, Flare Boost, and Poison Heal all have a clear and present drawback; as shown in the case of Snaelstrom, a mon that has to come in multiple times a game (we absolutely do) that is relatively easy to force out (we are), and is SR weak really does not make much use out of an item that precludes Heavy Duty Boots.
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Sticky Hold appears useful at first glance, but our physical frailty means that we really do not want to be taking Knock Offs. Zeraora, the weakest offensive user of the move, deals 46.3 - 54.6% to us, cleanly 2hkoing with Knock given even minuscule chip (see Volt Switch). Because of this physical frailty Sticky Hold is not really an ability we can use effectively.
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For different reasons any ability such as Rough Skin, Effect Spore, or Static is also a bad choice for us. We have 80/60 Bulk, this is not the kind of bulk that punishes U-turn, its the kind of bulk that gets 2hko'd by U-turn. Because of this physical frailty we are in no place to effectively answer U-Turn spam, thus these contact-based abilities are effectively the same as NCA.
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It should be noted that Neutralizing Gas excels in the long game; the longer the game goes on, the more Regenerates we deny, the more Intimidates we deny the higher its value. I think there are some abilities that could excel to the same extent (Regenerator itself, Magic Guard), but they're banned. As such I think we should focus on the short game as much as possible, by choosing an ability that provides an intense offensive boost, or makes it so that we can come in on Slowking for 100% free. Or, we choose an ability that expands our list of targets.
To reiterate; I think any secondary ability here should answer the question (in the teambuilder) of "Why am I running this instead of Neutralizing Gas" given the high power level of NGas itself. There's plenty of good abilities out there, but many of them simply don't provide enough of an impact, or a focused enough impact to ever really get used over NGas.
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Burn Mitigation:
In the short term the biggest threat that Slowking poses to us now is Scald Burns. This can be mitigated by a water immunity, a direct immunity to these burns, or an ability that makes the special set (on the off-foot thanks to 130 vs 115) competitive with the physical.
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Storm Drain is something that was brought up during the primary ability stage that is still incredibly relevant. We have the stats to make use of every single component of the ability, from the water immunity granting us Burn-proof Leech Lifes, to the SpA boost being incredibly relevant to our 115 SpA. If we want this mon to make a huge impact in the short term, this ability lets us do so with even one correct read. Def geared to mixed or special sets though.
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Sheer Force is a weird option, but directly targets our strongest Bug Stab in Bug Buzz, potentially boosts coverage moves, and honestly, is something we can easily target to affect the special side, or limited physical attacks only, providing meaningful set diversity. This is nice. Punk Rock is in the same vein, affecting Bug Buzz just as much, and has a far more limited list of targets, meaning we'd have a load more freedom in movepool if chosen. One small upside it has over Sheer Force is that it doesn't "require" a Life Orb, not that Sheer Force really does. Both of these abilities are very interesting ones that could allow for a physical Ngas set and a special non-Ngas set.
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Shield Dust is the weakest here, and honestly, I think its weak enough that it wouldn't see use over NGas basically ever. Realistically its way easier to just run NGas 28 + Slowking and get it in that way instead of running Shield Dust 28. Failing that you just run Ngas + Heal bell. The lack of impact compared to even Storm Drain means that I doubt it would see use.
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Short Game
Download has been mentioned a bit here, and while its def relevant, and would def boost the short game, it does worry me a tad; +1 attack for free on a mon with 130 attack is a good way to break well, basically everything. Yeah, it sorta forces mixed, but like +1 252+ Atk Life Orb Heracross Megahorn vs. 252 HP / 60 Def Equilibra: 199-234 (48.7 - 57.3%) its ridiculously strong. I could see this working, but its a very strong option and we'd have to strongly curtail a lot of movepool options to ensure balance just cause of that potential nuking power.
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Compound Eyes is an ability I sorta like because it makes a lot of cool options workable, but I do worry about like, what reasonable options it actually effects. Taking Megahorn from 85% to 100% is nice, but its not something you're gonna ever run over Neutralizing Gas, taking Dragon Rush from 75% to 97.5% is nice, but its not something you're gonna ever run over Neutralizing Gas. So basically, I just sorta wonder, will we get even a single option that justifies use over Neutralizing Gas?
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Competitive and Defiant are very nice options that give us a clear niche over Ngas (beating up Defog), play for the short game, and thanks to punishing Defog indirectly punish Pivoting spam. I like these, though the meta is very Rapid Spin centric atm, which makes the feasibility of this option a bit iffy. Potentially punishes Astro a bit? but like at -1 def Astro beats us, so not really.
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Rattled is also cool, and underused, but has similar issues to the contact abilities; while we can take 1 U-turn or Knock, we only take 1, and a one time use ability that gives +1 speed is nice, but isn't amazing.
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Expanding List:
Motor Drive is the only electric immunity that I think actually makes pivots think twice about clicking Volt Switch, as its the only one that makes us able to really threaten Zeraora and Krilowatt. For that reason I support this ability. I think there could be minor worries about it being OP, but like, if this mon is broken at +1 speed then we got other things to worry about. I consider this a high power but useful option that meaningfully expands the list of strats we check.
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Thick Fat is a bit of a silly option and may give the artists a stroke, but it meaningfully expands the list of mons we can check to include standard Astrolotl (the mon doesn't run Dclaw), Rotom-Heat (all the time), Krilowatt (all the time), and Volcarona. If you notice, that list, barring Volc, is entirely made of pivots, and because this ability directly targets a list of pivots that we otherwise don't answer all that well, I think its legitimately a really good option here.
Edit: I'm fully on the Thick Fat Train:
Thick Fat targets a specific set of pivots that I think we'd love to answer but currently cannot. Astrolotl, Krilowatt, and Rotom-Heat. It offers limited, but still useful utility against others including Volcarona and Smokomodo. Despite giving a significant boost vs Ice-type moves, we still die to Icicle Crash from Syclant. Because it offers a vastly improved matchup against a set of faster pivots, I think its a very relevant tech option that provides a meaningful contrast to the more offensively-inclined Neutralizing Gas.
vs Astrolotl (Pivot):
2x0 Atk Astrolotl Fire Lash vs. 0 HP / 0 Def Thick Fat Yanmega: 174-205 (55.9 - 65.9%) -- guaranteed 2HKO
2x252 Atk Astrolotl Fire Lash vs. 0 HP / 0 Def Thick Fat Yanmega: 217-256 (69.7 - 82.3%) -- guaranteed 2HKO
vs Krilowatt (Pivot):
252 SpA Life Orb Krilowatt Ice Beam vs. 0 HP / 4 SpD Thick Fat Yanmega: 109-130 (35 - 41.8%) -- guaranteed 3HKO
vs Rotom-Heat (Pivot):
+2 8 SpA Rotom-Heat Overheat vs. 0 HP / 4 SpD Thick Fat Yanmega: 168-198 (54 - 63.6%) -- guaranteed 2HKO
vs Syclant (Pivot):
252 Atk Choice Band Syclant Icicle Crash vs. 0 HP / 0 Def Thick Fat Yanmega: 290-344 (93.2 - 110.6%) -- 62.5% chance to OHKO