I might as well summarize my final thoughts of a slate too.
Rough Skin is a great ability which I wrote about already. Pairs really well with revenge killing without pumping damage into stats, can actually recover back later in the match after taking big hits thanks to speed, etc etc etc. Above all it is incredibly simple to tackle and I think we would do a good job here.
Multiscale is something Ive changed my opinion on. Please read this bc the framing of this ability I think does matter as to whether its a good choice or not. Originally, I felt off about the idea of a more passive lugia-style "im using multiscale to pseudo-nullify your boosts and then i'll force you out while i recover or cripple/phase you" because I felt like preserving the hp on the mon would feel overall awkward and recovery would be forced when taking a hit most of the time, meanwhile offensively I found it hard to imagine it actually threatening bulky offensive setup mons out which would awkwardly suggest having a utility move just for the purpose of crippling, and then suddenly the 4 moveslot syndrome kicks in and it doesnt provide anything in terms of playing mid-game like hazards or item manipulation, or we waste our excellent dual stab. A more passive multiscale route I imagined playing on the back foot a lot, and in a gen with less recovery pp it would just exacerbate this weakness further by putting its longevity on a timer. And I do still feel like this is a bit awkward.
But recently, I have been thinking a lot about mons like Mega Latias as inspiration for a very fast, offensive tank that uses its typing as invaluable checks to certain things but also very much focusing on meaningful damage output. To a lesser extent, Zamazenta offers something similar in the current metagame but skewed incredibly offensively (and one-time usage), while Mega Lati would offer 2 or 3 very threatening attacks while backing it up with recovery, and sometimes slot in a useful utility like defog or twave. For me, this could be a very effective route for Multiscale and if this wins then an offensive route should imo be looked at very seriously and not considered anti-concept- after all, this was one of the leading mons in the creation of the concept. This suddenly makes dual stab feel much more meaningful, 4mss is improved drastically and providing offensive pressure helps with its 8pp timer and reliance on recovery- whats more, being a cleaner in its own right grants it an additional purpose that makes it a more capable choice for teambuilding. Id also like to add that Dragonite is putting in a lot of work in its defensive set thanks to 134 attack/100 SpA- this is the most translatable example of multiscale that we have (I dont know if lugia really translates its walling set to OU, ubers is weird and sometimes things that are bad work there, like defog/toxic/wisp/rest giratina.... plus i think its only seen on FULL stall?) so I think we need to be very avoidant of passivity here. With this reframing, I really like this ability.
Honestly- these are the only two abilities that I really feel deserve to be picked for this CAP for primary. Im gunna list some abilities I dont like:
Flame Body/Static: as already stated by others, I agree that these are feelsbad abilities. Matches our concept excellently, but I really just dont like them or their effect on the metagame.
Cursed Body: Im grouping this with the above, having lost many games to cursed body procs at the wrong time. While in a 1v1, cursed body feels relatively harmless, on crucial turns it can win/lose you the game in a way that is different to static/flame body's ability to just spam recovery to fish for crippling effects. Getting the 30% cursed body proc to instantly stop a sweep after playing the whole game to enable your one chance to set up and finish the game is just miserable and can't be played around. Sometimes, you'll knock out a cursed body mon also and get disabled, allowing in a sweeper who will be able to take advantage of this to set up and end the game. The steps/repositioning required to avoid the 30% chance is even more ridiculous than for contact abilities as this activates on every move, and ultimately disable shenanigans are so lame that I seriously question what you'd gain from putting this into the metagame in the first place.
Toxic Debris: An ability that can improve an RU/NU mon to OU. Not only that, but the step up from rock/poison suicide lead to water/fairy wall with recovery is just absurd. I cannot stress enough how much this is unnecessary for the project and how freely this mon can litter the game with tspikes when the most popular spinners are hit super effectively and most popular poison type is not grounded. Even if these additional things weren't true, I would still be against this ability for how insanely powerful it is.
Bulletproof: An excessive ability that turns our amazing typing into the best typing matchups in the game. Im not afraid of making something like this in projects, but I want to explore fast walling tactics for this project, not create a laundry list of things we hard counter. We arent clamouring for more switchin opportunities.
Filter: Bit like bulletproof, not looking to improve matchups here. Filter is even more awkward as it does not guarantee even flipping matchups.
Shield Dust: Not sure how I feel about this one. Im not afraid to put a Covert Cloak on my water type to deal with garg, and... ultimately this ability provides a lot of use against midground mons and not much to match the concept. So Id rather do something else so we can explore it during the process and hold a cloak if its helpful.
Lastly, some abilities that I like but I think fit the role of secondary ability (or realistically nca depending on how excellent the primary is):
Pressure: Great concept-relevance here. Has a few useful scenarios. Ultimately does not take a big role in the success of its current abusers.
Natural Cure: There's not that much status in the tier. Getting hit by a sludge bomb is probably the most likely place to get statused, and that isnt something we want to happen. Rest + Natural Cure is honestly a bit shit in modern mons. But this could shrug off a burn from Ace/Pult and T-wave techs.
Mold Breaker: A nice list of small interactions, aside from the whopper which is defog vs gholdengo. I dont wanna polljump to assume that this will get defog though, so overall this is a small, relevant ability.
Compound Eyes: Hasnt been brought up before but an accuracy boost helps with choice of water STABs and improves some fast supportive moves like wisp to guarantee hits which makes them much more clickable in an otherwise dangerous scenario if you miss (think back to cinderace wisping tusk or lando for instance). But its minor outside of granting ridiculous moves like sleep and also assuming wisp is a polljump.