The Proxy (Slenderman Killer Concept)
I've seen Slenderman mentioned a number of times on the forums and while I think that would be cool, I always questioned how he'd be implemented because of how different his approach is. Then I remembered a section of Slender: The Arrival that was actually a lot like DbD; In the third chapter "Into The Abyss" the player needs to activate generators while running away and flashlight blinding Kate "The Chaser", a survivor gone insane and now under Slender Man's influence. So instead of playing as Slender, make one of his Proxies the killer, and allow Slender Man to operate as a passive power in a similar way he does in his games.
The Proxy - Now, the beautiful thing about The Proxy is it doesn't even need to be Kate "The Chaser", which allows the devs to be creative on the killer themselves regarding design, backstory, and loadout. The only aspect of licensing would be Slender, sort of like how Ghostface could be an original person, it was the mask that carried the weight. It turns out there are several proxies who the Slenderman uses to influence victims as needed, so it ties in just fine: https://theslenderman.fandom.com/wiki/Proxy
Slender Man - as mentioned above, I suggest Slender Man operate as a passive or active "power" or something of the sort. Slender Man is a supernatural being who only seems to reveal himself to his targets, taxing their sanity in his pursuit. Like The Doctor, his main niche is Madness, maybe even only being visible to the designated Obsession and actively pursuing them alone one at a time. However he's used, I picture him behaving in-game in a similar manner as he does in his own games: sporadically appearing to survivors (maybe even the Obsession exclusively) and looking at Slender will rapidly raise the survivors Madness (or "Insanity" if that makes it easier to code in conjunction with the Doctor's Madness) and once the Madness/Insanity is maxed or if Slender is physically touched, the survivor is treated to Slender's signature close-up and immediately placed onto an unoccupied Basement Hook.
Weapon - Crowbar (or anything really, preferably something distinct from the other killers)
Power - this varies depending how exactly Slender as either an active or passive ability would be implemented and who the devs decide who The Proxy is exactly. This whole idea kind of just dropped on me while I had some down time at work so I haven't really fleshed it out. I considered something like a Syringe Jab that would cause an Intoxication effect like the Clown as well as building Madness/Insanity.
(Possible) Perks: (note that I'm just throwing ideas and seeing what sticks)
I See You - When a survivor fails a skill check within 32/40/48 meters, their aura is revealed for 5 seconds.
Trepidation - survivors within your terror radius recover from status effects 20/25/30% Slower.
Hex: White Noise - Survivors become Oblivious while performing actions (repairing, sabotaging, healing, searching chests). After ending the action, the Oblivious effect lingers for 1/2/3 seconds.
Survivor - The Survivor could potentially be completely original like The Proxy, or it could be Kate from Slender: The Eight Pages (before she became a Proxy herself) Lauren from Slender: The Arrival or Hallie Knudsen from Slender Man the (albeit poorly received) film.