Take for example-

Nemesis Armor/Helm

To craft: Valkyrie Set + Fallen Set + Heavenly Iron Set + 3 celestial ore + 3 gremlonium + 30 eternal orbs of alchemy

Lots of normal defense, lots of shadow defense
Health bonus = +8, same as ancient plate set
Status resist: Curse, Fire, Poison, Negative Shock
global ASI: low on each piece
global CTR: low on each piece
Damage VS Fiends: high per piece
Sword damage: med per piece

This is about the power level of black kat, but it's a bit more defensive. It's the same rarity of black kat, as it requires a similar investment to be made, and it's arguably better for the shadow lair FSC. It would still be a five star item.

The idea here is that if we just buff armors then they'll replace black kat etc. and we don't want to do that, because black kat's rarity makes it special and players have already invested in it. No putting the cat in the bag again. Rather, we let more cats out of the bag, and suddenly there is diversity in armor and play style. The trick is to make them rare enough.