I think it could be a really cool idea if implemented properly!
If you mean “a literal item to prevent damage” I think it’s a cool concept that could abide by a set of rules similar to what @Arthadaw mentioned above where they function against specific forms of combat, but if we’re just discussing “temp health but maybe not temporary but maybe functions like energy-shields in Halo”, I’d say we just call that “armor.”
The reason I say that is only to make it more thematic and, honestly, there’s still probably a better term for it. I think it can add to tension in a lot of ways and make it fun/exciting where it functions similarly to armor in Killing Floor. While it exists, it absorbs a large % of damage while, simultaneously, reducing that damage.
Armor absorbs 60% of all damage taken, reduces that damage by half
100 damage vs 100 health = 100 damage dealt, dead
100 damage vs 60 health + 40 armor
60 damage goes to armor, armor reduces it by half, 30 damage dealt to armor
40 damage hoes to health, 40 damage dealt
20 health and 10 armor remaining, alive!
The amount of armor would *always go down more quickly than health (for balance reasons) and it also prevents a character from having 1000 armor and 5 HP because you still die when hit. These are, of course, made up ratios… but it does allow for a party to “heal to full” but have 0 armor and have everyone look at one another and say, “Well, if this goes pear-shaped, we might just die due to lack of armor.” It also makes healing a well-armored character super effective, sorta like healing a high DR character in Vermintide.
Maybe not an opinion others share, but I like it a lot. TL;DR: yeah, some form of defensive stat would be cool but I don’t want regenerating Halo-shields.
*Correction to above: Armor should have a lower maximum than health so you will always run out of it first OR you tweak the ratios so that armor has less DR/absorbs a larger percentage of damage.