Make an optional warning sound when you health is low.
It could be something as simple as a small screen shake with a distinct sound to let you know hey you're under 20% health or something. It could be something optionally that was annoying like the continuous beeping of Zelda type games when you're under a certain health.
Sometimes when you're in the heat of the fight you're not staring at your health bar or even feel you have the time to glance at it lest you get hit and die at times. A simple sensory indicator other than the health bar would be nice.
I can GUARANTEE that a beeping, shaking, or a little fairy saying "HEY!" will be used to signify low health.
Perhaps make the health bar "pulse," with the pulse fading after it expands outwards? Better than hearin' MASTER, REPLENISH YOUR HEALTH AT THE EARLIEST OPPROTUNITY.