I'd like to make a little suggestion for a new slender boss, which should remind some of you bad memories : the Winter Lantern from Bloodborne.
Model showcase. Head close-up.
OH GOD IT'S EATING ME ALIVE !!
The concept here is to make this beautiful creature a stalker/chaser boss. It works as the following : like other stalker bosses, the Winter Lantern builds up static whenever it sees you or you see it. What's new is it causes the player to take damage increasingly (starting from 5, max 10 hp per second) when the same conditions are met. It also chases you at a slow rate.