Once at 0 hp, character rolls a CON save. On a fail, dead. On a success, they take one point of CON damage, one level of exhaustion and are
dying (crawling on floor in agony, disadvantage on all rolls). Every round thereafter they must make a CON save or die. First aid returns character to 1 hp but they retain the CON damage and exhaustion. Prevents characters from popping up completely fine after healing. Getting to 0 hp has a consequence! Not as brutal but feels a bit more real than those arbitrary 3 death saves and instant KO at 0 hp