Hit Dice and Magic

I have written before about finding more uses for Hit Dice in 5th edition D&D. Here are a couple more ideas.

These make more sense if you consider a HD to be some manifestation of your will. Normally you use it to pull your own body back together from injury, but why not spread it around?

Wizards Copying Spells

Instead of spending 50 gold per spell level on rare inks and such to copy a spell into a wizard's spellbook, spend 2 HD per spell level.

You can split the effort across multiple rests. (1st-level wizards must do this.)

Specialists only need to spend 1 HD per spell level for spells in their school.

This seems like a reasonable extension of the wizard's will. I also tend to play in resource-scarce settings where there's never an opportunity to buy or make these inks. Also makes wizards a bit squishier, or opens up interesting possibilities around periods of vulnerabilities for wizards.

Instead of Costly Components

I've never really been a fan of the component system for magic since 3rd edition. That's probably a different post.

Require the caster to expend HD instead of requiring material components with a gp cost for spells. Most spells which require costly components are creating some kind of lingering effect or awareness or imposition of will - if you think of spending HD as imbuing something else with a sliver of yourself this works out fine.

Quick look at the spells with costly components in the SRD, I would break them down like this:

• 1 HD: Arcane Lock, Augury, Divination, Find Familiar, Guards And Wards, Illusory Script, Magic Mouth, Magnificent Mansion, Nondetection, Programmed Illusion, Project Image, Teleportation Circle, True Seeing
• 2 HD: Arcane Sword, Circle Of Death, Clairvoyance, Continual Flame, Find The Path, Glyph Of Warding, Greater Restoration, Identify, Legend Lore, Magic Circle ,Magic Jar, Raise Dead, Revivify, Stoneskin, Warding Bond
• 3 HD: Awaken, Contingency, Forbiddance, Forcecage, Hallow, Heroes' Feast, Holy Aura, Instant Summons, Planar Binding, Reincarnate, Resurrection, Scrying, Shapechange, Simulacrum, Symbol
• 4HD: Clone, Gate, Sequester

Then there are are a few weird ones:

• Astral Projection: 2 HD per subject
• Create Undead: 1 HD per corpse
• Imprisonment: 1 HD per HD
• Secret Chest: Breaks all the rules. At a wash, the creation of the miniature chest takes a HD, and the creation of the large chest takes another, which it keeps. If you lose the large chest you lose a HD. Adventure onto the ethereal plane to get it back.
• True Resurrection: 1 HD per HD 

Your wizards will look at this change like some kind of godsend, until an expedition doesn't go as planned, or an assassin sneaks into their home following the theft of a spellbook.