Below are some set bonus ideas for armor sets that don't have one yet:
Bone: Render undead mobs neutral AND/OR Make the player undead (inverse the effects of Instant Health / Instant Damage and Poison / Regeneration)
Coal: Armor pieces are usable as furnace fuel AND/OR automatically provide fuel to furnaces near the player
Emerald: Reduce villager prices
Ender: Looking at an enderman doesn't aggravate them (like a pumpkin) AND/OR save the player from falling into the void
Glowstone: The player emit light AND/OR give the glowing effect to the player or attacked entities
Lapis: Remove or reduce the lapis cost when using an enchanting table
Nether: Allow the player to walk on lava
Obsidian: Make the Heavyness set bonus also affect the Slow Falling effect
Paper: Give the Slow Falling effect to the player AND/OR allow the player to walk on water
Prismarine: Give the water breathing effect to the player AND/OR give the effects of aqua affinity and respiration enchantments AND/OR make guardians neutral toward the player
Quartz: Increase the player attack damage
Slime: Make the player bouncy (every block acts like a slime block) AND/OR reduce fall damage
Stone: Give a less intense Heavyness effect (the bonus of the obsidian set) AND/OR give the slowness effect to the player (and increase the armor and/or toughness of the armor set to compensate)
Wooden: Make the player burn indefinitely when on fire (need to be extinguished with water or similar)
Describe your idea
Below are some set bonus ideas for armor sets that don't have one yet: