This isn't really about health bar design per se... but I had an idea about showing what I'd call "tainted health" on somebody's healthbar. Tainted health would essentially show the amount of health that will be affected from damage over time abilities.
For instance, a player has 500 health. They hit a teemo shroom that will do 250 damage. Their health bar recolors 250hp at the end of the bar to another color (or perhaps a darker shade of the current color.) This is the amount that will decay over time from the sum of all debuffs currently on the player (poison, ignite, liandry's, etc.)
This would be similar to a user getting a shield from an ally, except in reverse. It shows how much health you WILL lose if you do not get healed/cleanse the DoT abilities. At later levels in the game a teemo shroom can half health you, but you won't necessarily realize just how much damage it will do until it's done all of it. It could allow for some counter-play for strong abilities like this... also ignite.
Has Riot thought about something like this? What is their stance on it? Is it possible this could be tested on the PBE to see if people enjoy it?