Ok so, since the announcement of Spelljammer™ for 5th Edition, I've been working on a homebrewed setting that has elements from various media sources, including but not limited to; Titanfall, Warhammer 40000, Eberron, Gears of War, and Halo. At it's heart it is a Exploration and Kingdom Building setting focusing on ranged combat and using cover for combat. I'm calling it The Grand Frontier. One of the first things I made was a Regenerating Energy Shield. Check it out.
Regenerating Energy Shield (R.E.S.)
Wondrous Item, common (requires attunement by a creature with at least two hit dice, any armor or clothing)
This wonderous item is a thin but flexible metallic playing card covered in arcane runes. For this item to work, you must attach the R.E.S. to any armor or clothing by pressing the R.E.S. against the armor or clothing for at least 10 minutes. Thereafter, the R.E.S can not be removed unless you detach it as an action or the armor or clothing is destroyed. Not even an antimagic field causes it to fall off.
If an armor or clothing requires attunement, you must attune to the armor or clothing before you can use it. However, the armor or clothing and the attached R.E.S. count as a single magic item with regard to the number of magic items you can attune to. If you remove the R.E.S. from the armor or clothing, your attunement to the R.E.S. ends. In addition, the R.E.S. does fall off the armor or clothing if you end your attunement to the R.E.S.
While attached to armor or clothing, the armor or clothing gain the follow properties:
Shield Pool. The energy shield is a pool of Temporary Hit Points known as a Shield Pool. The shield pool is made by dividing Hit Dice between a Hit Point Pool and a Shield Pool. For every Hit Dice you assign the Shield Pool, the average of that Hit Dice (rounded up) increases the Shield Pool's Temporary Hit Point Maximum. (Example: if you assign 2d10 Hit Dice it has a max of 12 Temp HP, 6 for each d10.) The Hit Dice assigned to the Shield Pool do not count towards your Hit Point Pool.
At least one HD must be assigned to each Pool. Your Constitution Modifier is always added to the Hit Point Pool per Hit Dice regardless of which pool the Hit Die is assigned to. Spells and effects that grant Temporary Hit Points are applied to the Shield Pool up to the Shield Pool's max, all points over the max are wasted (This is an exception to the "Temporary Hit Points do not add" rule). You can redistribute the Hit Dice once per day after a long rest. Hit Dice in the Shield Pool can not be used to heal HP during short rests.
Regeneration. The shield regains Temporary Hit Points equal to the creatures Proficiency Modifier every round until it reaches max Temp HP. This rate is doubled if you have half cover or hidden, tripled if you have three quarters cover or are invisible, and you regain five times the proficiency bonus when in full cover or are out of combat. Always take the highest bonus granted from this feature, they do not stack.
Vulnerability. The Shield Pool's Temporary Hit Points suffer double damage from Lightning Damage. Additionally, If the Shield Pool's Temporary Hit Points suffer Lightning damage, the Shield Pool does not benefit from its Regeneration property until the start of the next turn. The vulnerability only applies to the Regenerating Energy Shield and not the creature.
The Request
I'm still not sure how balanced it is, but I don't think it is overpowered. But that's me, and it has not been play tested yet. That's where the community comes in. If you could share this around and give me some feedback I would greatly appreciate it. I'll also answer any questions about it in the comments. Until next time!