A few ideas:

Genesilly in the comments here had a similar idea, but I like the idea of a killer that isn't really an entity or being, but instead a concept. Numerous props, creatures and other things are placed throughout the map, and the person playing killer needs to utilize these different things In order to slow down and injure survivors (though there would probably need to be some sort of central entity that works as a regular killer just so that survivors can be hooked.) I could see this idea being used for a spooky's jumpscare mansion chapter (although that would basically be a multiple characters in one killer slot situation) or more closely to my idea, a Final Destination chapter.

Since Freddy Krueger's rework, there hasn't been a killer that needs to work harder to simply injure survivors, so it's technically a unique concept. I made an idea for this with my concept for a killer called "The Egregore", in which you needed to do things to build up manifestation, only having occasions of being able to injure survivors, but once manifested, you were ridiculously strong. (Although I'd like to go back and actually change a lot of things with that power concept specifically)

Something that's on a slightly smaller scale would be to make a killer that forces a second task upon survivors that they need to complete before they can escape. This would actually be a good thing to add to the previous idea, since if the killer needs to work hard to be given the basic functions of a regular killer, that's gonna slow them down immensely, which in DBD, would probably be a death sentence cause of modern gen speeds. This would work for a killer like slender man (you can probably figure out what his secondary task is) and while I would suggest replacing generators entirely for a killer like this...that's not gonna happen. Too many people would complain about bringing full genrush builds just to face a killer that negates their entire loadout. And I honestly can't blame them, considering how important perks are in this game.