Dying
When a creature is reduced to 0hp they gain the dying condition. While dying you are prone, and your movement speed becomes 5ft. While dying you cannot take actions, bonus actions, or reactions and must make a death saving throw at the end of each of your turns. Any creature can move through the space of a dying creature, treating it as difficult terrain if the dying creature is of the same size or larger.
If a creature takes damage from any source while dying they gain a failed death saving throw. Critical hits on a dying creature instead inflict two failed death saving throws.
This dying structure still allows a player character to be 'part of the fight' even while at 0hp, rather than being incapacitated, entirely. They can try to crawl away from enemies, shout warnings, interact with the environment, etc. But they can't attack, cast spells, or otherwise engage with the combat in a more mechanically meaningful way than they currently can.
However, you can invent situations where a dying creature -is- mechanically useful (locking a door, pulling a lever, setting off a trap) but vastly less mechanically useful than an active PC on the field.
It also creates the chance to say your last words, beg for mercy, call out for your mother, or whatever else in narratively appropriate for your character as they're dying... briefly.
You could even combine it with a variation of the Doomed Condition of A5e for fun and profit.
Doomed
Once a creature has failed three death saving throws they gain the doomed condition. While doomed, a creature still faces the same restrictions as a dying character, but is beyond hope of all but the most extreme magics. You become immune to magical healing, and become a valid target for resurrection magic. Revivify can only end the doomed and dying conditions if it is used within one minute of gaining the doomed condition. A creature that is doomed will fully die within 2d12 hours, or at any time within the 2d12 hours that the creature chooses to perish by giving up their will to live.
If a doomed creature takes damage from any source, they die immediately.
More dramatic death scenes for player characters or important NPCs. It also allows the players to leave the BBEG doomed and dying on the floor of their throne room, only for one of their lieutenants or loyal minions to seek the help required to restore them to health and vigor... And similarly allows doomed players the chance for reinforcements to come save them when the BBEG leaves them to die in the misery of their own failure.
Basically creates a more interesting fail-state, I think, with more room for drama/narrative play.
