I believe that Pyuking Muku brings up a really interesting idea: a Pokémon with a lot of neutralities and few weaknesses turning its physical neutralities into resistances by boosting its Defense seems very pro-concept.
With this idea in mind, I would like to submit Steel/Flying.
1. What useful type resistances/immunities does this type have that mesh well with Diamond Storm?
Many Fighting, Ground, Steel, and Water types will want to switch in to a potential Diamond Storm to minimize its damage and waste its PP. However, they won't be able to do crippling damage in return, as Steel/Flying is neutral to Water and Fighting, resists Steel, and is completely immune to Ground type attacks. Resisting Flying is also a big deal, allowing CAP31 an easier time switching into and countering the meta's Flying types. Being immune to Poison further counters Books, and having a 4x U-Turn resistance and a 2x Bullet Punch resistance is never a bad thing.
2. Does this type come with any additional benefits that are particularly appreciated by CAP 31? (ex. Stealth Rock resistance, immunity to a status, etc.)
Steel and Flying makes CAP31 all but invulnerable to some of the most common means of wearing Pokémon down, those being Toxic and Spikes. Being immune to Sandstorm and Sticky Webs is nice, too.
3. What weaknesses does this type have that can be effectively patched by a Diamond Storm Defense raise?
Having a Fire type weakness is a huge deal that can be well dealt with by scaring out said Fire types with Diamond Storm. Unfortunately, an Electric weakness isn't one easily fixed with a Diamond Storm boost, given the type's predominantly Special lean.
4. Does this type seek to use Diamond Storm offensively or defensively? Or does it leave this more up in the air?
This type seeks to use Diamond Storm offensively, I think.
5. How does this type complement Diamond Storm coverage-wise? If this type does not have STAB on Diamond Storm, how does it justify using the move and how much value does the extra STAB coverage provide?
While the Steel typing doesn't add much offensively, the Flying typing allows CAP31 to hit Fighting types like Arghonaut for super-effective damage. Furthermore, Diamond Storm hits Flying types and Fire types super-effectively, and Electric types neutrally, providing valuable coverage against things CAP31 needs to beat and its checks.
The Steel/Flying type has many resistances and few weaknesses, as well as several neutralities, being Rock, Fighting, Ghost, Dark, Water, and Ice. I picked Steel/Flying because 4 of those 6 neutralities are mainly physical, and can effectively be turned from neutralities into resistances by doubling CAP31's Defense with Diamond Storm.
A few types I was also considering were Steel/Bug for having even more neutralities that can be turned into resistances, Rock/Grass for also having many neutralities, Diamond Storm STAB, and excellent offensive synergy, and Ground/Flying for its incredible coverage with Diamond Storm and useful resistances and neutralities. However, I decided against them for a multitude of reasons, including how none of them actually resist Flying, making it harder to counter Flying types, Rock/Grass still being weak against Steel and Fighting types, and Ground/Flying just striking me as Landorus but with actual usable Flying STAB and a more accurate Stone Edge. I believe that Steel/Flying strikes the best balance, but I still very much like these 3 other types and hope we get to see a little bit of discussion on them.
Personally, I'm not a fan of Electric types as, while they may counter Flying types really well, I feel like outside of Rock/Electric (which I love as an offensive typing but HATE as a defensive typing), an Electric type would often rather use its Electric STAB to fight Flying types, especially since those moves will almost certainly have more PP.
With this idea in mind, I would like to submit Steel/Flying.
1. What useful type resistances/immunities does this type have that mesh well with Diamond Storm?
Many Fighting, Ground, Steel, and Water types will want to switch in to a potential Diamond Storm to minimize its damage and waste its PP. However, they won't be able to do crippling damage in return, as Steel/Flying is neutral to Water and Fighting, resists Steel, and is completely immune to Ground type attacks. Resisting Flying is also a big deal, allowing CAP31 an easier time switching into and countering the meta's Flying types. Being immune to Poison further counters Books, and having a 4x U-Turn resistance and a 2x Bullet Punch resistance is never a bad thing.
2. Does this type come with any additional benefits that are particularly appreciated by CAP 31? (ex. Stealth Rock resistance, immunity to a status, etc.)
Steel and Flying makes CAP31 all but invulnerable to some of the most common means of wearing Pokémon down, those being Toxic and Spikes. Being immune to Sandstorm and Sticky Webs is nice, too.
3. What weaknesses does this type have that can be effectively patched by a Diamond Storm Defense raise?
Having a Fire type weakness is a huge deal that can be well dealt with by scaring out said Fire types with Diamond Storm. Unfortunately, an Electric weakness isn't one easily fixed with a Diamond Storm boost, given the type's predominantly Special lean.
4. Does this type seek to use Diamond Storm offensively or defensively? Or does it leave this more up in the air?
This type seeks to use Diamond Storm offensively, I think.
5. How does this type complement Diamond Storm coverage-wise? If this type does not have STAB on Diamond Storm, how does it justify using the move and how much value does the extra STAB coverage provide?
While the Steel typing doesn't add much offensively, the Flying typing allows CAP31 to hit Fighting types like Arghonaut for super-effective damage. Furthermore, Diamond Storm hits Flying types and Fire types super-effectively, and Electric types neutrally, providing valuable coverage against things CAP31 needs to beat and its checks.
The Steel/Flying type has many resistances and few weaknesses, as well as several neutralities, being Rock, Fighting, Ghost, Dark, Water, and Ice. I picked Steel/Flying because 4 of those 6 neutralities are mainly physical, and can effectively be turned from neutralities into resistances by doubling CAP31's Defense with Diamond Storm.
A few types I was also considering were Steel/Bug for having even more neutralities that can be turned into resistances, Rock/Grass for also having many neutralities, Diamond Storm STAB, and excellent offensive synergy, and Ground/Flying for its incredible coverage with Diamond Storm and useful resistances and neutralities. However, I decided against them for a multitude of reasons, including how none of them actually resist Flying, making it harder to counter Flying types, Rock/Grass still being weak against Steel and Fighting types, and Ground/Flying just striking me as Landorus but with actual usable Flying STAB and a more accurate Stone Edge. I believe that Steel/Flying strikes the best balance, but I still very much like these 3 other types and hope we get to see a little bit of discussion on them.
Personally, I'm not a fan of Electric types as, while they may counter Flying types really well, I feel like outside of Rock/Electric (which I love as an offensive typing but HATE as a defensive typing), an Electric type would often rather use its Electric STAB to fight Flying types, especially since those moves will almost certainly have more PP.