Purpur blocks can be made from popped chorus fruit of which the regular version can heal like food so maybe a full set of purpur armor can slow down hunger?
Amethyst shards can be used to make shulk sensors which detect movement so a full set of amethyst armor can do the same or detect enemies somehow?
Maybe an ice armor full set that freezes enemies (not bosses) when they touch you?
Maybe a full set of flint armor that allows you to set fire to anything you punch?
To add to the ideas of @justsomebodynew as well as adding some of my own:
Purpur: Could instead make you immune to the levitation effect OR allow you to descend while having the levitation effect by sneaking OR give you the levitation effect while sneaking OR make every food you eat teleport you (like a chorus fruit)
Amethyst: Could instead make sculk sensor detect you from further away OR allow you to transmit sound to nearby sculk sensors (like an amethyst block placed near a sculk sensor would do)
Ice: Could instead just give slowness to every mob you attack or that attacked you OR could give you ice physics (every block you walk on would act as an ice block) OR could also make you immune to freezing damage (from powder snow)
Flint: Could also set on fire everyone that attacked you
Sculk: Could give you echolocation (by giving the glowing effect to every entity making a sound near you) OR could allow you to not activate sculk sensor or shrieker at all, even when walking on them
Cactus: Could come with a built-in thorns OR could inflict damage to every entity touching you (like a cactus)
Feather: Could give you the Slow Falling effect OR give you Elytra flight
Honey: Could allow you to slide down every block (like when sliding down honey blocks)
Cake: Could give you the saturation effect
Glass: Could greatly increase the damage done and received (x10 or x100) and would have only 1 point of durability and give you no armor
Cobweb: Could allow you to climb walls (like spiders) and cobwebs don't slow you down or give slowness to every mob you attack or that attacked you
Describe your idea
Purpur blocks can be made from popped chorus fruit of which the regular version can heal like food so maybe a full set of purpur armor can slow down hunger? Amethyst shards can be used to make shulk sensors which detect movement so a full set of amethyst armor can do the same or detect enemies somehow? Maybe an ice armor full set that freezes enemies (not bosses) when they touch you? Maybe a full set of flint armor that allows you to set fire to anything you punch?