M_Natas
Hero
I tried to do the exhaustion thing at my table. You can gain a one level of exhaustion to get the benefits of a short rest or gain two levels for the benefits of a long rest (without HP and Hitdice regaining).When things like this have been brought up before I remember being convinced that it really makes the game not fun to have to decide how much closer to death you want to be to do things on a regular basis.
One place I've wondered about using it is when people run out of their daily or short rest abilities. What if you usually only do that maneuver once because it's so taxing... but if you reach down and push it you can do it again. Where reaching down and pushing it is the hit point expenditure. (I could also see exhaustion instead of hit points too).
Nobody ever used it.