Edit: Now using the term Mortal Wound to replace Failed Death Save.
In the thread on whether 5E is viable at high levels, I remarked on a set of rule changes I was considering as my party moves from Tier III into Tier IV and will also include whenever I start my next campaign. As I'd like further comment, I thought to bring it into its own thread.
The goal here is to increase flexibility between day long missions and multi-day or even multi-week missions, without requiring the rules to arbitrarily change based on the mission type. The two primary ways I accomplish this is making dropping to 0 HP much more dangerous, Failed Death Saves more permanent (already in my campaign they reset on short rests rather than instantly when healed) and decoupling Healing and Ability Regeneration into different types of rests.
I welcome any thoughts and insights to strengthen and tighten this tweak.
Death Saves
1. When you drop to 0 HP, make a death saving throw. DC is 10 or the number of hit points below 0 the attack took you, whichever is higher.
2. When you fail a death save, you also gain a level of exhaustion and a Mortal Wound. You can never have fewer failed death save marks than you have Mortal Wounds.
3. Mortal Wound's are removable by extended rest or one at a time by Greater Restoration or similar magic. On a Long Rest, you can make a DC 20 Heal check to remove a Mortal Wound, failing this check also prevents you from removing a level of exhaustion during the rest. Failure by 5 or more increases your exhaustion level by 1.
4. When you have a Mortal Wound, you cannot gain Temporary Hit Points from any source.
Resting
Breather - 5 Minute Rest where you can spend Hit Die (also available during any other type of rest)
Short Rest - 1 hour rest uninterrupted by combat resets short rest class abilities.
Long Rest - 8 hour rest uninterrupted by combat resets long rest class abilities and removes 1 level of exhaustion. You also regain 1 spent Hit Die if you are level 1-10 or 2 spent Hit Die if you are level 11-20. You do not regain Hit Die from the rest if you have a level of exhaustion or a Mortal Wound prior to starting the rest.
Extended Rest - 7 day rest uninterrupted by combat resets HP to Max, returns all hit die, and removes all exhaustion levels and Mortal Wounds.
Edit - Special thanks to @Helldritch for feedback from the original thread