How important is STAB given that we want to utilize Diamond Storm? Would certain roles appreciate STAB on it more than others?
I think the more offensive CAP 31 is, the less necessary STAB will be for Diamond Storm to be used.
As others have pointed out, Rock is a very solid Coverage option for some typings.
Especially those, that have trouble with flying and fire types will appreciate rock coverage.
Obviously any typing, that lends itself as great coverage, is also very capable as a STAB type. Which means that we will have a very open playing field if we want to use Diamond Storm on an offensive Mon.
Now Diamond Storm is a powerful move in its own right, but I see it being less effective on its own or as coverage, the less powerful CAP 31 attacking stats are.
Once you reach a certain threshold, Pokémon start running more Spammable and utility focused attacks, as their damage output is just not high enough to threaten high damage anyway and the additional utility has a higher impact throughout the game.
It’s secondary effect grants it some utility in some situations, but I am not certain, that it would be enough to supplant a utility move like Knock Off or Stealth Rock or Will O Wisp in its moveset, because while its typing is solid offensively as stated, Diamond Storms low PP make it less spammable than other moves - on a defensive Mon it probably would be most comparable to Gyro Ball on Ferrothorn, which mostly is there to ensure having a high power attack against fast and frail attackers but, given Ferros nature, rarely if ever is used over any of its utility options in Knock Off, Hazards or Leech Seed.
Additionally, with the exception of Body Press, mainly defensive mons like Ferrothorn, Clefable or Arghonaut won’t run Attacking moves without utility unless they are STAB moves.
Tldr: The more offensively inclined CAP31 is, the less important STAB will be compared to its overall synergy with other (attacking) moves.
What types synergize well coverage-wise with Diamond Storm?
Others already have mentioned the most obvious options between the legendary edge/quake, rock/fighting and fire.
I do like the mention of Electric, as Diamond Storm has some tough match ups into Bulky Water types, especially Toxapex, Arghonaut and Slowbro as well as into Corviknight, will Rock actually threatens Landorus, the main Ground Type.
Grass is another interesting option, that appreciates rocks ability to deal with flying, fire and Bug types, while threaten Grounds, that aren’t Lando.
Last I’d say Fairy might be interesting as it can hit Garchomp and Arghonaut as well as threaten Urshifu, which all look to be good switch ins to Diamond Storm atm.
Edit: I feel like a typing with Strong neutral Coverage like Ghost or Dragon can be very valuable, since Rock has several important targets in the meta and can force the switches, that these types can capitalize on
In general Id say, that rock isn’t as easy to justify on a two attacks set as it once was, and often appreciates two additional coverage types, but in combination with some of the above types, forms a Trifecta, that threatens large parts of the meta Super effectively.
Do we value narrowing the playing field with regards to what role CAP 31 will inhabit by giving it a more specified typing?
I don’t think we have to at the moment.
Id rather focus on Rocks and Diamond Storms intrinsic offensive and defensive value and which types synergize best with it.
Rock is an iffy typing into defensive roles specially and I think, that we might narrow the playing field too much if we took a decision now.
Diamond Storm has a powerful secondary effect that can improve CAP 31's matchup into physical attackers. What are common physical attacking types that CAP 31 can leverage Diamiand Storm's secondary effect against effectively?
I think we should look at Chromeras process here a bit. We’ve already seen, that halving the power of certain attacks in one turn means you can turn a resist into a 4x resist, a neutrality into a resist and a weakness into a neutrality.
I still think that turning a neutrality into a resist and a weakness into a neutrality are more powerful than improving an already existing resist.
Now the effect isn’t guaranteed so it’s not as clear cut as it was with Chromera.
What attacking types exactly we can take advantage of hinges on which typings we chose ultimately.
I do think that Rock itself has an interesting relationship with this effect, since it’s generally poor defensively, with some glaring defensive issues and only a few reasonable resists.
What I want to say is, that creating a Rock type Pokémon, that suddenly is neutral to ground and fightings or resists ice types is hella interesting.
I believe, that it will be more important to look at the matchups into mons, that want to switch in on Diamond Storm just to find themselves in a position, where they become less effective as an answer due to its effect.
This means Ground, Steel and Fighting types, as well Rock neutral Physical Pivots.
An obvious target is Landorus, as the defense boost helps deal with Earthquake and Diamond Storm still hits neutrally.
Other mons include Ferrothorn, Melmetal, Grachomp, Slowbro (if it’s Body Press), Kartana, Urshifu R, Arghonaut, Collosoil, Revenankh, Buzzwole and Skarmory.
That said, Urshifu R and Arghonaut ignore the defense boost anyway, which means they seem like hard to tackle candidates, while Melmetal, Garchomp and Kartana can be difficult to deal with even with a Neutral Mon, unless you already have above average bulk.