When you or a teammate spots an enemy player, and their health dips below 20%, their icon on the minimap will start to flash red. Teammates below that same health threshold would flash blue. There’s no need to make the player icons a different size, shape, or base color of the icon, and the flash should not glow beyond the circular boundaries of the icon. The frequency of the flashes would increase as the players dips closer and closer to no health; to avoid it becoming a nuisance. This would be a good way of alerting players on both sides as to who is the weakest target, or which teammate is in danger of dying; if communication is not possible within a match.
I understand that this will likely, in some respects, lead to kill stealing (if you care that much) but it also allows for players to passively aid fellow teammates either by getting the heal on them before the die, offering support to kill an enemy player, or simply to chase down a weak opponent. This would also be a less invasive approach to helping healers find out who they need to help out.