Just a few ideas
Idea 1: (TL;DR - Surv on hook LOSE Madness vs Doctor)
After a game I just played (somehow never came up before) - could a hooked survivor vs DOCTOR who WAS in Madness LOSE madness so they're able to heal someone unhooking them? Game I had was a tricky situation where I couldn't heal to full (medkit ran out) and killer was guarding the hooked survivor BUT the only exit gate open meant that (as I was only survivor remaining) if I got close enough he WAS coming after me... so after managing to juke him and double-back to get the unhook thinking that surv could top me up to full health (then I could take a hit for them) it turned out they were still in madness so couldn't do a thing and now both of us were injured with killer almost back.
Idea 2:
Add puddles for sound and/or rain and/or lightning? Could really make the game that bit more atmospheric, and while the sounds could hinder the killer the lightning flashes could really catch survivors out. Dunno, maybe not the right game for this but could be worth trying out.
Idea 3:
Make skill checks on gens WAY HARDER (smaller circle?) while killer in a chase. While NOT in a chase make killer able to walk faster than normal OR see if no survs in a certain radius when say 20m from a gen?
Idea 4:
New killer - Medusa, look at her and she at you = turn to stone for 3 secs. Not given any further thought, was just something which could maybe be worked with.