Some ideas

Ideas by Danish14803

Mobs
Shadows are a mob that can spawn if the player digs too deep underground, at about Y-32 downwards.

Red ghosts: can go through walls, can fly, behaves like a wither except it fires red poison.

Blue ghosts: similar to the red ghost, except that it can spawn water blocks.

Green ghosts: more helpful than the red or blue ghosts. Green ghosts will add saplings and wheat to your world.

Rainbow slimes: appear at night, drops rainbow essence.

Corrupted armor: a pet, uses "dark banisher" that does 8 damage.

more mobs coming soon

Items
Dark shards: used to make dark tools and dark armor, along with corrupted armor.

Rainbow essence: used to make rainbow tools and rainbow armor

Red poison: like lava, but does 3 more damage.

Dark tools: all do 8 attack dmg.

Rainbow sword: does 8 attack dmg.

Rainbow tools: same as dark tools.

Dark armor: better than diamond. All are crafted with 4 dark shards around a diamond chestplate/helmet/leggings/boots, resulting in that armor piece.

Rainbow armor: better than dark armor. Rainbow armor is crafted similarly to dark armor, with 4 rainbow essence around a dark armor piece.

I will add more stuff later.