What if, after reaching 0 HP, you make it a chance that the next attack will render them unconscious as well as do whatever critical damage is associated with it? Roll a D10, if it lands on 1 you're KO'ed, unless the enemy character is aiming for KO, in which case you roll a smaller die? I guess it depends on how you're controlling what critical damage gets done to the player.

I think you'll just have to have another depleting pool for elemental powers...not much else you could really do there.

Well, in the MU once they die they technically get teleported to the Red Star sans armor and equipment. And afterlife minisagas aren't uncommon in tabletop RPGs. You know, where they reach the afterlife and undergo a trial to get an extra life? In this case it could be tricking the Kestora or finding a way off the red star. It would basically be that, but they'd lose all their equipment, but they'd still be the same character. Maybe some memory loss if you want to make it super frustrating and lame. =B

As for elemental energy, pfft, good luck with that one.

Subdual/lethal damage is a mechanic in D&D. If a character takes Lethal Damage that puts them at or below 0, in 4e at least, they are unconscious and begin Dying, where they die if they fail three/four saving throws, depending. Subdual damage just KOs people.

Not a clue how to do fatigue, though. Maybe use Elemental Energy as an ammo stand in or something?

Various random ideas that I thought of over the course of a school day. I do not promise that any of these are good.

1: Each type of power has a power level, and the character has a power level max thing. Every time you use an elemental power the characters power level goes down by one. (ehh this is basically another way to have a elemental energy bar. )

2: Dice pool? Every character has a certain number of die for fully charged EE. Using one in a roll pulls it out of the pool. if you have no dice, you can't cast anything. (this would work alright in a burning wheel esque system but idk about here.)(so this way a fireball is just 1d(x) whereas a nuva blast would be (y)d(x). better powers would be fatiguing you faster.)

(this still basically boils down to another depleating pool.)

3: Stat check. Every time you use a power you check to see if it begins to fatigue you at all. at a certain number of fatigue levels you can't cast anything. More powerful moves have a higher chance to fatigue. for example, fireball would be 1d20 or whatever, fatiguing on 1-2. nova would be instant. delayed fireball would be fatiguing on 1-7. Something like that.

(problem with this is wow the difficulty. It would allow for editable powers though? like "add a delay effect for +2 fatigue chance. concentrate it for more damage for +1, ect. This sorta freeform spellcasting could be super cool) 

4: (completely irrelivent) every character could have a slot for the UDD. Each one would be written in for how the character see their relationship with that? so a toa could say that their Unity was their team, their duty was to protect said team, etc, whereas a skakdi would be against it and then the players would get bonuses due to roleplaying to their character.

pretty much just stolen from Burning wheel but man that sounds super cool for this.

I do believe that is all I got. *shrug*

Maybe one of these will help you out in some way. 

I like the ideas and I'll definitely have to see if any will fit.
 
As for UDD, I've already got something in place for working with those that I'm pretty stoked about.

Takuma Nuva

I'm working on my own Bionicle RPG right now, and here is how I'm going to be handling elemental fatigue.  The idea is based off of Star Wars RPG.  In that system, each character has two health bars (HP and Vitality).  When taking hits, a character's vitality is depleted before receiving killing damage.  The twist for Jedi is that they have to spend vitality points to use the force.  I'm going to do the same thing in my game.  A Toa will have to tap into his or her vitality points to be able to use elemental attacks.  Elemental attacks will be really strong, but they will also cost vitality.  This cost will force players to think strategically and also prevent certain attacks from being spammed.  Vitality will be recovered by resting while killing damage will require healing.  If vitality is depleted, the character will have to do an endurance save to prevent being knocked out.  If HP is depleted, the character will have to have immediate medical attention or the character will die.

Pathfinder makes you start dying at 0 HP. You will bleed out by 1 HP every round after that, and if you reach a negative value equal to your Constitution score, you die. In D&D I believe they use -10 no matter your Constitution. Every round you also round the die to see if you stabilize. If you do, you stop bleeding out, but are still unconscious until healed. While someone is dying, you can opt to kill them.

BIONICLE happens to have very sturdy characters, so I don't think I'd have a problem with "only" fainting after being tossed through a wall at 3 HP. But if I choose to get back up and then get tossed through the same wall once more, I think that would justify an injury. I think a BIONICLE RPG could do well with saying that any damage beyond that which puts you at 0 HP is ignored, but that additional damage can be inflicted after this point to critically injure or kill you. Perhaps there is a period of a few rounds after this first occur where you do not get this "saving grace", so that if you choose to heal yourself and keep fighting you can now go into negatives if you're not careful.

Say, when you take damage that goes beyond your HP, you get 3 points of "fatigue", get put at 0 HP, and become unable to do much of anything. This would be very similar to D&D's "downed" state, except maybe BIONICLE characters can manage to stumble away from the fight without fainting. For every round after this, you roll to see if you may remove one of these fatigue points. For as long as you have any fatigue points, you are not protected from damage that would exceed your HP, which means that you can get more permanent injuries. Any hit that actually puts you into negative HP will cause a real injury and possibly unconsciousness. The type of injury can be somewhat directed by the attacker, allowing - as an example - a Toa to put his foe unconscious but not dead. If you are healed to regain HP, you may take normal actions in combat again, but the fatigue must be removed the slow way, and you are now at risk of getting downed instantly.

D&D also has some rule about massive damage - if you take more than a certain amount of damage in one hit, you have to make a Fortitude save or else be instantly downed. Certain things also have an "instant kill" switch on them, when you could not realistically survive in any way.

For elements, I kinda like the typical mana/power points approach. Each ability costs a certain amount of points, but like D&D Psionics you could possibly spend a few extra points to "charge" an ability to be stronger. This would work nicely for Toa, I think. When your points run out you must wait for them to recharge. It's simple to understand and fits well with the canon version of elemental energy in some form of battery. Having 3 point stats shouldn't be much of an issue - one for HP, one for your element, and one for your mask/equipment power.