This is an idea for a new boss, any tier, probably 3.. since we only have vanaduke.

The Horrific Slime of chaos.

Changes into a different base damage and defences every 6 seconds.
He'll alwasys have high normal defence but his secondary defence will change between shadow, elemental and piercing.

He will spawn from his bulk other minor chaos slimes that can be of any type of damage/defences (they dont change all the time though)

Recipes:
Sealed sword -> Chaos Axe
Rock Salt Bomb -> Chaos Detonator
Prismatic Alchemer -> Chaos Alchemer

Chaos axe changes every 6 seconds into a different effect, the charge ability is a bigger area of effect sealed sword type charge.
The different damage type can be piercing, elemental , normal or shadow. with a possibly random affliction including curse.

Chaos Detonator detonates with a random effect each time it detonates, you can end up with any sort of vaporizer or a shard bomb , graviton or knockback

Chaos alchemer bits split like any other alchemer but each split can end up with a different bullet type, dealing elemental, shadow , normal or piercing.
The charge ability fires a bolt that bursts at the target like a chaos detonator but with much reduced radius. also a very slow charge time.