Sort of inspired by Umbra-Lunaris' post, I had a sort of vivid clump of ideas for a revival system. Let's jump right to it shall we?

Health Share
Least likely of the bunch but still worth mentioning. Players use a percentage of their health and the same amount of pips are given to the player who is being revived. This can be anywhere from 10% to 50%.
PROS: Limited resource, not infinite revives (however it can be quite a lot), sense of teamwork.
CONS: One bad player can break down a party via needing a lot of revives, thus taking away the party's health as a whole. Sometimes accidental revives happen when in combat and walking over a friendly player's body.

Life Essence
One of my personal favorite ideas - add a new rarity called a "life essence" into the drop table. This item can only be used to revive teammates, and will only revive them to full red health (30 pips max). They are activated like old health revives.
To keep things in line we would reduce the drop rate of sparks of life by 60%. Life essences would then drop 45% more often than current sparks of life do.
You can also purchase life essences from the supply depot at 15 life essences for 150 energy.
PROS: Limited resource, not infinite revives. Players would want to revive other players as they have no use for the rarity themselves. Would allow newer players to play with veterans on higher difficulties because veterans wouldn't feel bad reviving the newer player.
CONS: Will be harder on solo players since sparks of life would become more rare. Newer players will also have more trouble until they get a good supply of sparks/essences. Might make the game a bit "too easy" for parties, as you will probably have hundreds of team ERs in your collective pockets.

CPR
Yup, CPR. This idea is loosely similar to the Castle Crashers revive system. You run up to the fallen knight, interact with the attack button (second attack confirms, shield you can exit the revive), and doing so will revive them after X attacks. Doing this will not take away any of your health, however it does make you vulnerable for the duration of the revive. If you land the attack buttons correctly (little minigame based on timing) then you will grant your revived player more health when they revive - spamming attack buttons will revive them at 1 pip per tier. Perfect timing revives players at (33% + 2 pips per tier) health.
PROS: Gives a great sense of teamwork, gives a sense of urgency to reviving in combat, health revives have a tier system. Does not hurt a party if you have a weak link.
CONS: Limitless revives. If not explained too well many new (and old!) players would simply revive players at minimum health, and go down again fast. Not something you can do in the middle of combat, and enemies may stand on your teammate's body. Players may also try to abuse revives by killing themselves and having another player revive them with more health. We'd probably have to remove the ER if this was implemented.

Defibrillators
Shock paddles! Defib units would drop in the clockworks as a consumable item. When a player goes down, you walk near them and activate a defib unit. The defib unit would then deploy on the ground, shock enemies nearby, or if they are within 2 tiles of a player they will shock the player back to life with 50% health. This can jump to multiple players but only revives 50% health maximum (two players within 2 tiles will revive at 25% each if they are both down, if a player goes down next to a defib unit after it is spent all 50% health it will revive them for 5% health). After 7 seconds the defib unit will self-destruct, shocking enemies around it (even shock immune enemies!).
PROS: Extremely limited resource - up to a maximum of 12 revives per party if lucky. Perhaps add them to the consumable shop at Basil and provide a crown sink. Wouldn't dig into player's pockets often, and since it's very difficult to use to revive yourself (have to die within 7 seconds of placing the consumable) there's no reason to not use it on an ally.
CONS: Might be too rare. Players may not know it can be used for revives and instead drop them for attacking enemies. Would hog already precious inventory space (maybe give them a slot similar to the vitapod's). Not able to be found in places where you might really need them, such as Dreams and Nightmares.

Those are all the potential revive ideas I have off the top of my head. What other kinds of revive system could work in the game? What do you think about these ones?