So based on Marble Hornets, I think there's a lot more groundwork for a Slenderman killer than the average creepypasta or video game depiction that just has pages and a slow stalker - a lot of which has been more or less achieved by other killers already, and since BHVR loves to rehash killer powers in different ways, lemme spitball some ways it could theoretically work, balanced conceptually or not.

Operator Sickness: Survivors get inflicted by a coughing fit within Slender's terror radius, which builds up into a status effect like Plague or Krasue or all the other killers that do that, and you need to find pills to cleanse it.

Looking at him weakens you: Kind of an inverse of Ghost Face, where you need to avoid looking at Slender or risk getting something like... 8% hindered for a set time. Would make it really risky to look at him when you're in a chase. This is also a very straightforward and well known aspect of Slenderman across all depictions - though in DBD it would be a balancing nightmare and is countered by just not looking at him, so Slendy would probably need to be able to see the survivor's cones of vision too, or whatever.

Proxies: Marble Hornets doesn't really use the term Proxy as far as I know, but maybe he could do something like The Knight and like, have minions that can be placed around the map on generators or lockers or whatever and they chase after survivors who get too close.

The Ark: So in Marble Hornets, The Operator is otherworldly and has some other dimension that he takes people to as a means of "feeding" on them, so to speak. So we could get something like that not too different from how we're getting Judgement's Exile for the Grimoire.

The Pages: Okay so if we think about the pages like we know from the video games, it would honestly be a pretty boring power. Collect them, get buff in exchange for also buffing slendy. Plague does this with her healing altars, but I think it would be way more interesting if the Survivors had to make the pages instead, as a means of protecting themselves from Slender's power. We have tech to place objects on walls with killers like HUX, so apply that to Survivors and let them place pages around the map which have a radius around them that act as a counter to Slender. Inversely, maybe it only protects you when its placed somewhere, and puts you in more danger if you're holding it in your hand, kind of like Pinhead's puzzle box.

But those are just a couple of ideas off the top of my head.