Some various ideas occurred to me recently about setting hacks for Don't Rest Your Head. It might be more accurate to say that some of these are just different contexts for the DRYH rules, but that's just semantics. The interesting part is play.
Don't Lose Your Head
Yes, this is the Highlander setting. Although the setting is quite old (at least 22 years old, by my reckoning) it is still quite evocative and has potential for some great roleplaying.
Exhaustion becomes... Bloodthirst
For those who remember the first movie (we don't talk about the other thing that was allegedly a sequel) there is a remark made about the Kurgen that “all the killing has driven him mad.” This is a real phenomenon for soldiers, or anyone else who has to wield a firearm for money, and I think that there is plenty of scope for this to be part of the struggle for the Immortal. This is a human variant that is driven to kill in order to become all that it can be. The urge to seek each other out and battle to the last is instinct, and I think that this would be an interesting characteristic to keep in check throughout the story.
Madness becomes... Quickening
This stuff can do anything, and takes the place of a good plot wildcard. Does the Immortal need to run like the stag? Can do. Perform some crazy acrobatics while dodging a sword cut? Can do. Meditate to find another Immortal? Can do. Use it too much and the Immortal will gain permanent Quickening, meaning that a little of their humanity (Discipline) is lost and they are one more step removed from mortality. Lose enough discipline and the Immortal is a dangerous NPC with no humanity left, but with plenty of Quickening.
The only system hack that I would suggest is the starting number of Quickening boxes and the acquisition of more. Start the new Immortal with only two or three boxes, representing the limited control that they have over it. There are two ways in which this could increase: mentorship and taking the head of another Immortal. A mentor can increase it by only one box, no matter how long is spent under instruction. Taking the head of another Immortal, however, would add another box of temporary madness that the Immortal can use. As an example, Willam Mcleod starts out with two boxes when he meets his cousin/uncle Connor. After some training, William goes up to three temporary boxes that he can bring to any conflict. After taking the heads of two other Immortals, this goes up to five. So, for each conflict, he can bring up to five temporary dice of Quickening.