I've been toying with adding a new resource, "Wounds", which would track with critical hits. In my version HP are fatigue and "wounds" are meat. Basically, combat is a lot of fatigue with an occasional crit = wound (spells crit on a 1 if it is a save spell).
Anyway...In a current ad hoc version, HD + Con mod = Wound Threshold (how many wounds you can take). Exceed wound threshold and you fall unconscious and are dieing (normal death save rules apply).
My issue with my ad hoc house rule is at higher levels that is a lot of wounds you can take before going down. I've upped how criticals work (+1 for rolling a 20, +1 for bloodying, +1 for reducing to 0hp, +1 for specials like brutal crit feature, for 1-4 wound crit) but that almost seems too deadly at low level. However, this would help nicely...at low levels, you don't really face energy drain type effects, but at higher levels you do, so the drain represents your diminished ability to be wounded. Since, in my system, wounds are the "slow heal" resource (or ritual+gp heal), it can make high level play not seem a cake walk. I may have to up the number of creatures that do HD damage for it to balance right, but certainly a way out of my conundrum.