Time to get spooky in the most classical way possible. Ghosts mainly spawn in graveyards (although they can be found in other areas rarely) in small groups.

Visuals:
Ghosts are mostly white with varying colored eyes. They float and have no feet or legs.
Potential design inspiration:

Mechanics:
Ghosts will float towards and have a short aggro range (<10 blocks). They are immune to projectiles and take 1.25x damage from the Reapers Scythe.

Stats:
HP: 15
Damage: 4-5 (2x-2.5x hearts)

Drops:
Not entirely sure what they should drop yet-- I figure it could be a resource used for graveyard related recipes. Maybe a rare item that converts Zombie Villagers into normal Villagers?

They should drop Ectoplasm. Not sure what it would do though.

(by the way, you can already convert Zombie Villagers into normal Villagers in vanilla with a Splash Potion of Weakness and a Golden Apple)