I'll chip in with my thoughts.
Ground vs Fighting: So I'm grouping these two together. Why? Because in many ways, they're really quite similar in how they support the concept, and I think it's important to distinguish the more unique defining traits between the two. Looking at the posts in the thread so far (not to mention the discussions on PS), both of these typings have been paired with several other typings (they're both been paired with Ice and Fairy for example), and upon a brief inspection, it's not too hard to see why. Both offer a resistance to Stealth Rock, which is certainly nice for a Pokemon that's going to be pivoting around with Parting Shot. Both offer a super effective hit against certain Steel-types; however, the specific targets are what vary between these two, and I think that's perhaps the most important distinguishing factor to consider moving forward. Fighting gets a SE hit against Ferrothorn, a problem for VoltTurn teams due to its typing, bulk, and ability. Ground on the other hand offers some neat benefits, notably an Electric immunity, which blocks Volt Switch and provides CAP 22 with an opportunity to switch in and grab momentum, not to mention it has some nice two-move coverage with some of the other proposed typings so far. So again, we should definitely look more into specific targets with these two typings and analyze how well they synergize with other primary/secondary typings.
Fairy: Alright, something I keep hearing from multiple people in this thread is that "Fairy doesn't get good super-effective coverage, just neutral coverage in tandem with [x]-type" or something along those lines, and I'm actually genuinely confused by this sentiment, so I'm going to address it. Even if you're unfamiliar with the metagame, if you're willing to do just a small bit of research, you'll find that a quick skim of the CAP VR, usage stats, or even just watching some CAPTT replays shows that there are plenty of top Pokemon that are weak to Fairy. As a result, I'm not really buying into the argument that Fairy + (secondary type) is only good for neutral coverage and/or wouldn't be able to force switches very well; I'm inclined to believe the opposite. That being said, I think the next step here is to consider the unique threats that Fairy does offer strong coverage against in comparison to the other types being suggested. Are VoltTurn teams already capable of sufficiently pressuring said threats? Are there other threats that are a bigger issue that would be better addressed by a different typing?
Ice: Ice, like Fairy, is also a really nice offensive type to consider, hitting Ground-types like Colossoil, Garchomp, and Landorus-T super effectively, all of which threaten to put a damper on Volt Switch users, not to mention it gets good SE coverage overall. That being said, it does introduce a weakness to Stealth Rock (though this can be neutralized with a secondary typing, as seen in some of the type pairings suggested so far). Also, as a (presumably) speedy Ice-type pivot, it would naturally face competition from one of the top Pokemon found on CAP VoltTurn teams, Syclant. For example, if we went with Ice/Ground, that mirrors Syclant's existing coverage pretty closely. If Syclant is able to force roughly the same switches as CAP 22, as well being able to force switches with STAB U-turn (which Parting Shot obviously cannot do by itself), it becomes more difficult to justify CAP 22's inclusion on a team. Of course, there are plenty of other opportunities CAP 22 will have to differentiate itself later on in the creation process (ability, stats, AND movepool), but it's just something to keep in the back of our mind as we move forward.
Lastly...
On Bisharp: I've brought this up both on PS! and on IRC, and I'll bring it up again here. I feel like some people are overstating the importance of "beating Bisharp" with the typing. Of course, the risk of using Parting Shot on Bisharp switch-in is kind of a big deal since you have to deal with the threat of a +3 Sucker Punch the next turn, so creating a Pokemon that just lets Bisharp in for free really isn't the best way to address the concept. However, the way we should be addressing it should be just that: making sure Bisharp doesn't get in for free. We don't necessarily have to be able to switch into Bisharp reliably, and there are many ways we can ensure it's hesitant to switch into CAP 22. Having SE STAB is obviously one of them; but there are other methods that can also be explored later (namely during the movepool stage with coverage/utility, not gonna go in-depth here since typing is our sole focus at the moment).
Now, this isn't meant to imply that Bisharp isn't an important factor for us to consider, it just shouldn't be our main focus-- there are a multitude of other variables for us to be considering. Think along the lines of common stops to prevalent VoltTurn Pokemon and strategies, as well as how this Pokemon is going to synergize with its presumed teammates. Which kinds of Pokemon will CAP 22 be drawing in with its typing and using Parting Shot on? Which specific teammates would benefit from the Atk/SpA drops provided by this move? Any particular setup sweepers? Any additional momentum grabbers?
This is all the time I have right now, and there were plenty of typings I didn't touch on, so I'll probably be commenting more later. Think about the questions above when it comes to making or arguing for or against typing suggestions-- and don't be afraid to take it a step further either! We'll be able to understand and analyze the benefits and drawbacks of each typing more deeply this way, which will hopefully lead to a more informed final decision when it comes time for the poll.
Why exactly are we building this for a voltturn team? Is there any particular reason this has to cater to voltturn cores? Haven't we done cores enough already? I apologize if this is not what you intended to say, but in the section about fairy you state we should talk about how fairy type benefits voltturn.
As for Bisharp, as long as CAP22 isn't Bisharp bait it'll be fine. So as long as it isn't weak to it, and as long as it has any move, STAB or coverage, that can deal a decent enough chunk of damage to Bisharp, Bisharp won't want to come in.
Which kinds of Pokemon will CAP 22 be drawing in with its typing and using Parting Shot on?
I'll answer this with my proposals Water/Fairy and Electric/Fairy.
Cawmodore is a potential switchin to Water/Fairy, unless CAP22 is special attacking, and especially not if it is scald spamming. Cawmodore has Volt Absorb, so switching into Electric/Fairy isn't a big deal for it either, but the benefit of this is you can easily pivot to your Skarmory/Quagsire/etc. The thing is, whether they switch into cawm or not, whether they switch out of cawm or not, you still gain momentum.
Mollux is fine switching in to either with its water immunity, 1/4 fairy resistance, and high bulk. Mollux will definitely be an answer. If choice specs mollux, that special attack drop will hurt it's damage by a lot, and you can easily switch to your sweeper. Most Mollux sets are pretty offensive, so this is an easy way to cripple it until it switches, and the utility sets are mostly setup bait for your sweepers.
Landorus-Therian would like to pivot into Electric/Fairy, and as annoying as Landorus-t can be, hampering its damage and allowing yourself an easy switchin to your Landorus check is a huge help. Even shaky checks can be full counters with the damage reduction from parting shot.
If Water/Fairy Landorus won't want to switch in, but baiting Landorus-Therian isn't too important, but it is helpful.
Krillowatt wouldn't mind coming in on either set. It has electric moves for water, and ground moves for electric, so with either combo, it's easy shrimp bait. Krillowatt's damage was never that impressive in the first place, even with a life orb, but the parting shot debuff makes the threat value extremely low, allowing for an easy setup on your sweeper.
Necturna wouldn't mind coming either, and necturna sets can vary so much, having a pivot to bait it and switch out is pretty helpful.
Volkraken (volcanion too tbh) is pretty bulky and wouldn't mind coming in on Water/Fairy CAP22, and Volkraken boasts a high special attack stat that's hard to switch into, especially with the analytic boost, but with parting shot's debuff, but once it's power is cut with parting shot, it's not so scary anymore.
Electric/Fairy could also bait out Crucibelle and switch to the proper answer, and crucibelle isn't so powerful at -1.
Pyroak wouldn't mind its STABs either way with its bulk, although parting shot debuff makes the SD set pitiful when it can only get to +1, and seed set is easily dealt with through pivoting.
Megagross would probably be a pretty common switchin, especially pre-mega to block the debuff, but metagross is still dealt with through the proper switchin. If post mega, the debuff is on both stats, so slowbro is safe to switch in even if it runs GK.
Ferrothorn would have an easy time switching in, but you can also easily parting shot straight to your magnezone.
Heatran would happily switchin to Electric/Fairy, and if utility Heatran, it becomes straight-up setup bait after the debuff. If offensive, then with the debuff, a lot more pokemon can safely tank it's hits and counter it than before. It's like a temporary assault vest for your colossoil!
Plasmanta is the obvious switchin here, resisting fairy and electric, and being immune to water, plus getting a boost off of it. Although it won't be setup bait due to taunt and haze, it is not very fearsme at -1 with only poison/electric/hidden power coverage, and can easily be dealt with by the proper switchin, which is most ground types lol. And if it does carry haze, baiting it out is actually pretty crucial for your setup sweeper's success.
Venusaur will switchin, and if offensive venusaur, parting shot debuff forces it out as soon as it comes in, providing an easy opportunity to set up your sweeper. If it's a defensive or utility set, you can switch to your aurumoth to quickly dispose of it (or set up quiver dance tbh bc theres no way it is staying in), or if you aren't worried about lech seed, just set up.
There's a lot more, but I've been working on this post on and off for too long.
Which specific teammates would benefit from the Atk/SpA drops provided by this move? Any particular setup sweepers? Any additional momentum grabbers?
Setup sweepers in general really appreciate this, especially the more frail ones, because even if the stat drops don't force the opponent out for a setup opportunity, they can actually tank a hit or two.
Aurumoth for one, is gonna love coming in on those weakened poison types and forcing them out while it sets up quiver dance (or even tail glow or dragon dance, if that's your cup of tea)
Tail Glow Syclant and Calm Mind Stratagem are some of the most frail set up sweepers, and with the aid of parting shot, could actually viably set up, despite still being weak to priority.
Naviathan's natural bulk already aids its setup, but if it comes in on a weakened mon, it's not taking much damage and it easily has a set up opportunity.
Mega Charizard X is in the same boat.
It also lures a lot of mons with ground weaknesses, so if a voltturn core is your thing, which colossoil already fits well on, colossoil is a good and powerful answer to these.
Excadrill would like to come in and set up a swords dance on these poison types. As would most steel types, like scizor.
Cawmodore has something better than swords dance though. Cawmodore would have a field day setting up on the likes of Crucibelle and Plasmanta (immunity to both it's STABs lmao, and with the perfect bait too)
(I'll add more later)
