I've been thinking of a hallow boss called the Neon Eye. It would function as a more difficult EoC, with its most noticeable characteristic being that it always stays in the same position on the screen as your character moves, making its attacks difficult to dodge
I've been thinking and I got an idea myself : The shadow of the unicorn's horn TSOTUH for short this is a boss with phases 1 and 2. 1 is regular attacks such as rush, knockback , slow ,etc etc phase 2 is like duke fishron's 3rd phase screen is blurry, and dark. a small light only comes from TSOTUH but he cloaks every time you hit him and (s)he will not attack , You must guess TSOTUH movement (Left right up down diagonal horizontal) and finish him off. He will do this 3 times before healing 2/8 of his health and going back to phase 1 and this repeats. I say he/she should be after duke fishron but can be spawned after hard mode starts basically saying learn how to do duke fishron expert before taking this on.
I will go for a boss called Shine Worm, he has 30 segments, excluding head and tail, his main segments have 100 HP, his head and tail have 800 HP, and when one normal segment is destroyed, the worm splits into smaller worms,similar to eater of worlds. after the big worm is killed, the player will get the message The Hallow Worm Has Awoken! and then the Hallow worm will spawn which is his true form. He spawns color worms, which have 60 hp, and the big worm is the same as the boss before, except he deals 50 damage instead of 45, and each segment have 250 hp, and his head and tail have 1050 hp.