Another idea would be situational health/danger indicators...

1. The borders of health bars for each unit can change from black -> yellow -> red.
2. When a unit is not targeted, it will always be black regardless of current health.
3. When a unit is targeted by a unit/units, the game will compute how fast the targeted unit will die depending on the targeting party's DPS factoring in the targeted unit's health and both party's distance.
4. If the unit will die in 2-4 seconds, the borders turn yellow.
5. If the unit will die in less than 2 seconds/instantly, the borders turn red.
6. The effect is for normal attacks only and will not be triggered by invisible attacking units and spells.

Of course I don't know how plausible this is or how much of a performance hit computing what color the border is, but we look at health indicators to find out unit longevity right?