Alright, as promised, here's my opinion about some of the most talked typings:
Fire/Flying: This is my favorite combination at the moment. Flying-type offers useful resistances to Ground- and Fighting-type, the most common attacking types in the meta, which complements Fire very nicely, giving us a lot more opportunities to pivot in. It's also offers incredible offensive coverage without being too overwhelming, with Rotom-Heat and Stratagem alongside the more uncommon Tyranitar, Cyclohm, Krilowatt, and Dracozolt resisting the combination. As snake pointed, Heavy-Duty Boots is really a blessing for this typing, as it can completely negate its double weakness to Stealth Rock and additionally dissuades sets like Choice Scarf/Specs/Band, which might go against the concept. Now, Knock Off is undoubtedly a big weakness of this combination, as it would force us to take Stealth Rock damage, but I don't think this is too big of an issue, as we can plan around it by not focusing on trying to check threats with Knock Off like Clefable, Mandibuzz, or Colossoil, as Fire/Flying offers more that enough even with that additional restriction.
Fire/Dragon: This is the best way to take advantage of a primary Fire-type offensively, as it is resisted only by the very uncommon Primarina. Defensively, it's decent but nothing particularly impressive over other Fire-based typings, as losing its Ice- and Fairy-type resistances for an Electric-type resistance and neutrality to Water-type is kinda meh outside of the Zeraora matchup. I feel like this combination veers towards offence just a bit much for our own good, but its still one of the best options available, as the fact that it doesn't need any rea
Fire/Electric: This is another great offensive typing, as Rotom-Heat currently demonstrates and has great options that would allow us to spread burn and paralysis only using STAB moves. However, while resistances to Flying-, Ice- and Electric-type are great, it's still rater suspicious on the defensive side, as a x4 Ground-type weakness will make us offensively easy to check. I think its positives outweighs its negatives and could prove to be a very useful typing for CAP 27, so I'd also like to see more discussion on it and other possible combinations for Electric, which I think is very underexplored in the current discussion.
Fire/Dark: Given that Ghost is one of the most spammed types in the metagame, having one of the few types that resist it is obviously something we should look at, and once again, Fire compliments it very nicely by removing its Fairy-type weakness and adding valued resistances. in terms of raw offense and defense, this typing might not seems that good, as it doesn't offer as many resistances as Fire/Flying and is not as lethal as Fire/Dragon, but its unique qualities still makes it one of the most attractive options. I think that access to STAB Knock Off is a bit overrated, as almost any other typing could make use of that move with the right tools, but it's still another point in its favor.
Fire/Ghost: Ghost is always a solid option, as not only is a great spammable offensive type, but it also provides a valuable Fighting-type immunity, and Fire pairs particularly well with it, as Ghost doesn't remove any of its resistances. Its coverage is great, as is only resisted by Hydreigon, Tyranitar, and Crawdaunt. That being said, I'm not as huge on this combination as others, because I feel like the metagame doesn't need another Ghost-type attacker, but its still a solid option.
Water/Ghost: While I see that this typing has a lot of positives, I feel like Water-type in general is too passive in the current metagame, with many immunities and resistances being common, so I'd always end up preferring Fire/Ghost over it, which has many of the same resistances with more offensive presence. That typing might not be as good as a counter to Syclant, but it could still serve as an excellent offensive check to it, and I don't think its wise to focus on fully countering mons given that we're supposed to be primarily offensive in nature.
Poison/Dark and Poison/Normal: I'm putting these together because they share the same problem: lack of useful resistances. The first one only resists Ghost, Dark, Poison, Grass, and Psychic, but out of these, I'd only consider the first two valuable, as the others are currently very uncommon. Poison/Normal is pretty similar, having a Ghost-immunity and a Fairy resistance as its only noteworthy traits. As I said before, I think having several good resistances is much more useful than having few weaknesses. We're supposed to be primarily an offensive threat, so if we want CAP 27 to be able to enter the field many times per game, we should be abusing our resistances to find opportunities to switch in, not banking on our bulk to tank neutral hits.
Fire/Flying: This is my favorite combination at the moment. Flying-type offers useful resistances to Ground- and Fighting-type, the most common attacking types in the meta, which complements Fire very nicely, giving us a lot more opportunities to pivot in. It's also offers incredible offensive coverage without being too overwhelming, with Rotom-Heat and Stratagem alongside the more uncommon Tyranitar, Cyclohm, Krilowatt, and Dracozolt resisting the combination. As snake pointed, Heavy-Duty Boots is really a blessing for this typing, as it can completely negate its double weakness to Stealth Rock and additionally dissuades sets like Choice Scarf/Specs/Band, which might go against the concept. Now, Knock Off is undoubtedly a big weakness of this combination, as it would force us to take Stealth Rock damage, but I don't think this is too big of an issue, as we can plan around it by not focusing on trying to check threats with Knock Off like Clefable, Mandibuzz, or Colossoil, as Fire/Flying offers more that enough even with that additional restriction.
Fire/Dragon: This is the best way to take advantage of a primary Fire-type offensively, as it is resisted only by the very uncommon Primarina. Defensively, it's decent but nothing particularly impressive over other Fire-based typings, as losing its Ice- and Fairy-type resistances for an Electric-type resistance and neutrality to Water-type is kinda meh outside of the Zeraora matchup. I feel like this combination veers towards offence just a bit much for our own good, but its still one of the best options available, as the fact that it doesn't need any rea
Fire/Electric: This is another great offensive typing, as Rotom-Heat currently demonstrates and has great options that would allow us to spread burn and paralysis only using STAB moves. However, while resistances to Flying-, Ice- and Electric-type are great, it's still rater suspicious on the defensive side, as a x4 Ground-type weakness will make us offensively easy to check. I think its positives outweighs its negatives and could prove to be a very useful typing for CAP 27, so I'd also like to see more discussion on it and other possible combinations for Electric, which I think is very underexplored in the current discussion.
Fire/Dark: Given that Ghost is one of the most spammed types in the metagame, having one of the few types that resist it is obviously something we should look at, and once again, Fire compliments it very nicely by removing its Fairy-type weakness and adding valued resistances. in terms of raw offense and defense, this typing might not seems that good, as it doesn't offer as many resistances as Fire/Flying and is not as lethal as Fire/Dragon, but its unique qualities still makes it one of the most attractive options. I think that access to STAB Knock Off is a bit overrated, as almost any other typing could make use of that move with the right tools, but it's still another point in its favor.
Fire/Ghost: Ghost is always a solid option, as not only is a great spammable offensive type, but it also provides a valuable Fighting-type immunity, and Fire pairs particularly well with it, as Ghost doesn't remove any of its resistances. Its coverage is great, as is only resisted by Hydreigon, Tyranitar, and Crawdaunt. That being said, I'm not as huge on this combination as others, because I feel like the metagame doesn't need another Ghost-type attacker, but its still a solid option.
Water/Ghost: While I see that this typing has a lot of positives, I feel like Water-type in general is too passive in the current metagame, with many immunities and resistances being common, so I'd always end up preferring Fire/Ghost over it, which has many of the same resistances with more offensive presence. That typing might not be as good as a counter to Syclant, but it could still serve as an excellent offensive check to it, and I don't think its wise to focus on fully countering mons given that we're supposed to be primarily offensive in nature.
Poison/Dark and Poison/Normal: I'm putting these together because they share the same problem: lack of useful resistances. The first one only resists Ghost, Dark, Poison, Grass, and Psychic, but out of these, I'd only consider the first two valuable, as the others are currently very uncommon. Poison/Normal is pretty similar, having a Ghost-immunity and a Fairy resistance as its only noteworthy traits. As I said before, I think having several good resistances is much more useful than having few weaknesses. We're supposed to be primarily an offensive threat, so if we want CAP 27 to be able to enter the field many times per game, we should be abusing our resistances to find opportunities to switch in, not banking on our bulk to tank neutral hits.