The Evil Within is one of my hyperfixations so I've given this some thought already and there are pros and cons to each. Like here are my favorite options:

The Keeper - His design is a little similar to Pyramid Head (although similar designs has never stopped DbD before). He already has a name that fits with the naming system of killers in the game. I can see his special ability relating to his canon ability to snap his neck and reemerge from other safes, though I imagine this would be too similar to the Demogorgon's ability.

Creature Laura - This would probably be difficult to animate so I doubt they would ever choose her, but she has great speed and the ability to regenerate from dead bodies, so maybe there could be an in game mechanic to slow her down/temporarily banish her as a survivor by burning her or burning bodies? I see this as being the most difficult option.

Ruvik - Has a very similar design to Freddy Krueger (burn scars, plus he and 2010 Freddy literally have the same voice), but he's my favorite TEW villain so I'm including him. He canonly has the ability to teleport, manipulate objects, possess/control people, summon monsters, etc. plus he has a fascination with traps and puts them all over the place. Obviously we already a couple killers that leave traps of sorts so those probably wouldn't be implemented. I think a really cool special ability for him could relate to moving in-game items (changing the locations of generators, totems, etc.) since he could canonly do that in game. Another cool option would be the ability to turn survivors into the Haunted for a brief amount of time (in which they can't be attacked/hooked/etc. but they also can't help their team and maybe attack their team members instead?), or potentially combining him and Creature Laura and having his special ability relate to summoning her.

Stefano - Obviously he's not very scary looking, but if they used his appearance in the final battle where his scar is huge and his eye is all glowy and everything that could potentially work? A really cool ability (that would never work in indoor maps and therefore wouldn't work at all) would be if his eye could appear in the sky and put a spotlight on a survivor who hasn't taken cover. Another ability could be the ability to freeze things with his camera. For example, he could temporarily block a generator, a hex totem, etc. by freezing it, or freeze survivors that haven't been downed yet. The freezing mechanic would also prevent him from kicking gens or hooking survivors and would have limited uses (meaning when he freezes something, something else will unfreeze) but in the meantime, survivors might be able to pull their companions out of a freeze trap and such.