(I'm not sure if anyone already had similar ideas but here goes...)
In conjunction with 5e's customizable complexity flavour, the save against effect could also proc when
a)PC is reduced to below X hitpoints.
b)PC has taken a total of more than Y damage to hitpoints
c)PC is bloodied
d)PC is reduced to a surges worth of hitpoints (25% HP)
e)the attack deals more than Z damage
f)the attack deals damage more than the PC's surge value (reduce more than 25% of PC HP)
g)two or more of the above
Mix and match to taste.
In conjunction with 5e's customizable complexity flavour, the save against effect could also proc when
a)PC is reduced to below X hitpoints.
b)PC has taken a total of more than Y damage to hitpoints
c)PC is bloodied
d)PC is reduced to a surges worth of hitpoints (25% HP)
e)the attack deals more than Z damage
f)the attack deals damage more than the PC's surge value (reduce more than 25% of PC HP)
g)two or more of the above
Mix and match to taste.