These are just a few simple ideas and tweaks for existing armor sets:

Kat sets outside of the Black Kat set shouldn't require the Whiskers to craft, the Helmets don't need it, so the armors shouldn't either. These aren't gamebreaking sets, most are on par with existing armors.

Volcanic and Arcane Salamander should resist Pierce damage not Elemental. Both of these sets have damage boosts against enemies that deal Pierce damage but don't resist Pierce themselves. You're leaving yourself wide open defensively on a set that's supposedly "Specialist"... I'd imagine anyone calling themselves a specialist would at least know to equip the right defenses.

The Divine Veil and Divine Mantle should offer the same offensive bonus against Fiend, I think it's the only set in the game that offers different buffs on the armor pieces.

Vitasuit Deluxe should have a 5 star variant... in a similar issue to the old Cautery sword you end up with wasted player effort and time if they decided to turn their Solid Cobalt Armor into it because they will lately have to build an entirely new set to pass the Hall of Heroes mission for Vanguard. It's not a great armor as is so you could get away with giving it something fancy beyond an HP boost. Poison and Burn resistance seems fair for an armor aimed at keeping you healthy. (Perhaps even give it a helmet counterpart)

Radiant Silvermail lacks a helmet counterpart.

Boost Mercurial set to MSI Very High making it the choice for speed demons.

Nerf Chaos set down to DMG and CTR Medium. Weapon and Monster specialized armors should be able to outperform everything else at their job, Demo sets should be best for Bomb CTR, Sword and gunner sets should be superior for using swords or guns, specialist sets should be better at their specialization. Chaos should be a jack of all trades, master of none set. Something that can be effective in any situation offensively but never the best at it's job.

Nerf Black Kat set down to just Very High Damage but keep the MSI Med. Reaching maximum universal damage should be difficult to impossible and come with a steep cost for attempting to do so and even with the status debuffs the speed boost helps you deal with most threats.

Nerfing these two offensive sets will force players to expand their armory and spend more time playing the game to build more sets. Currently, you can focus on getting the Chaos set and ignore every other piece of armor in the game because it simple outclasses them all offensively with Black Kat being the only other contender in that area.