I guess these ideas count as unique spawns/minibosses

Idea #1: Sandman/Dreamwalker/Mister Doldrums
Description: If they ever bring Sleep-inflicting back into the game proper, this guy would fit right in... he's basically a sky blue Zombie in a starry cloak/robe who manages to look LESS menacing than the usual unthemed Zombie.
His breath attack is actually some sand he throws at you out of a bag and he prefers to run rather than fight. He can also set enemies to sleep and he's extremely passive, probably because he's simply the anthropomorphic personification of sleep and dreams in the Clockworks, like how Punkin King is to the harvest. He starts running when attacked and when you give him enough damage/wait long enough he just opens a portal and runs out on you.

Idea #2: Trevor/Percy
Description: A Black Gun puppy with X eyes depicted in a standing on four legs position instead of a sitting position like other gun puppies. He has a funnel on his head from which steam rises and a bunch of wheels on his legs. He appears in a special boss room with train tracks and switches and gates. At random intervals two of the gates will open, and he'll come rushing out of one and running along a path until he reaches the other gate, making a whistling noise as he rushes past.
The pathway is automatically set whenever he appears so that he has a clean path through the two opened gates. You can only damage him by hitting the switches to make him crash into blast cubes, shut gates, and other enemies. He is pulling along with himself a bunch of trucks with boxes and monster cages on them and he will randomly drop one every now and then.

Idea#3: Gremlin Mixmaster
Description: He looks like a T3 scorcher but swap the mask for a pair of DEUDLY shades and his giant back-pack is now a FRESH boombox with two "arms" at the ends of which are a pair of turntables. He uses MAD sound-wave attacks to Stun/confuse enemies and attacks in a rhythmic FUNKY manner. He makes Record scratching sounds when he gets WICKED hurt, or retreats backwards and whenever he shields an attack with a pair of records as his shields. He can unleash a special attack that causes all action to completely stop and everyone, knights and enemies alike, starts dancing while he drops a series of INCREDIBLY ILL JAMS on his equipment