Help me refine this concept: magic items that draw power from their users.

[Quoz]

A useful homebrew we used in a previous edition had magic items that drew their power from the person using them. This could be gaining scaling bonuses based on character level or more unique benefits like improving on class features. These weapons would have unique properties based on the character wielding them. For example, the Anyblade. This weapon could change shape into any sword or knife as the owner saw fit. In the hands of a barbarian it would add to critical damage, shifting its mass to the point of impact mid-swing. For a duelist it would allow an improved defense, changing in length to more effectively parry. And for a rogue it would improve sneak attacks, changing from a narrow stabbing blade after it had passed through armor into a barbed and serrated flesh-render.

I'm trying to refine the concept into well-defined and easy to understand magic items for 5e.

- wands and staffs no longer have their own charges. Instead the owner must spend healing surges to channel his own power through the item.

- weapons will have a list of potential properties like bonus to attack and damage, additional elemental damage, or increasing the save DC of a single class feature. When first attuning to the weapon or when gaining a level, the user may pick a number of options up to their proficiency bonus. Some may be taken multiple times, like attack and damage bonus, but never more than 1/2 proficiency rounded down.

- base model magic armor would follow the example of the Warforged racial armors, adding proficiency bonus into the AC equation. Advanced versions may have abilities that are activated by spending healing surges, such as the ability to deny forced movement or gaining resistance to an attack.

The important thing is that it feels like the items gain their power and magic from the person attuned to it. The magic blade they find in their first adventure is still useful right up through the end of the final epic quest, but is not overpowered from the beginning and respects the principles of bounded accuracy that are so integral to 5e.

[Vogie]

I definitely like the idea that magic items can use Hit die to recharge things, as they're an underused resource that isn't refreshed on a long rest, and have to slowly be regrown (with the exception of DMs that use the DING! rule on leveling up).

It also somewhat counters the Linear Fighter/Quadratic Wizard problem, as spellcasting classes as a whole have smaller hit die than their martial counterparts.

Some ways to use it on weapons & armor:

Blooddrinker - When this weapon makes a critical hit, expend a hit die as a damage die when rolling damage for that hit
Body-blocking - As a reaction, you can expend a hit die, and add the rolled value to your AC for a single attack
Entrench - As a reaction, you can expend a hit die, and add the value to a saving throw against being pushed or knocked prone
Bloodfeud - When this weapon makes a hit against a chosen specific creature type, expend a hit die as a damage die when rolling damage for that hit.
Heartseeker - As an action, you can expend a hit die and make a single ranged attack (with a ranged weapon or a thrown melee weapon) and add the die roll to the attack roll.
Elemental Resistance - As a reaction when you take elemental damage, you can expend a hit die to gain resistance to that damage for a number of rounds equal to half the die roll, rounded up. This is an unique effect, and cannot be stacked with multiple types of damages.