Hiya!
I'd probably just make a rule that allows full-casters (all of the Arcane ones; but not 'part time' ones, like the Eldritch Knight, or Divine ones like Cleric or Druid) to substitute 1 Spell Level regained for 5hp of "HD resting". Casters generally don't have quite as many HP's as others, so that makes it a trade-off. Making it 5hp also means it pretty much relegates it's use to very desperate situations...ones where one particular spell is REALLY needed again, AND the caster needs to likely spend/use 2 HD's worth of 'rest healing' to get a 1st level spell. I'd play it out as the caster taking the time to pour over his notes, hit the books, scribble formula, arcane symbols, and perform rituals of burning incense, chanting, or whatever....while everyone else is resting comfortable on their bedroll's getting a good nights sleep.
That said...it might just be an easier "fix" to just allow casters to write spell scrolls easier with a smaller cost...but have those spell scrolls "tied to the caster" (meaning nobody else can use them). Probably easier in the long run.
^_^
Paul L. Ming
I'm not 100% I have your concept, let me restate and see if I have it right. During a rest, HD spent can regain spell slots as well as HPs for full casters, at the rate of 5 HPs per spell slot level. This will result in more total spell slots per day, not replacing normal recovery.
I think this would be very powerful.
And common. But not common as in regaining a bunch of 1st level spells. I'm worried about regularly regaining 9th level spells. Well, all of the high level slots really, the ones the game already restricts you to just getting 1 of.
Blue said:Replacing 45 HPs worth of self-healing with healing from another caster is easy-peasy when it comes to having another game-defining 9th level spell. Even more efficient when there are several casters who heal via mass heals and keep all of their HD to regain spells. Having extras of spells of that level for the sacrifice of a few low level healing spells is broken.
Blue said:Even if perfectly balanced, additional flexibility on how to spend HD grants power - so this makes full casters more powerful. I have heard no one say "full casters are across the board weaker than non-casters" in 5e (or any addition). Granting them more power without a nerf is no good. So if it was balanced, you still need to nerf casters, otherwise it's just occasional more free power.
Blue said:Now let's get to other cases. Half caster archetypes will better be done with myulticlassing full casters (who can do this) with non-casters than the existing half-casters. And the non-casters have bigger HD so that helps recover.
CON is already more important for the low HD classes - +3 CON mod is a bigger % of your HPs if you have a d6 vs. a d10, plus the ever=present Concentration saves. It stays important here, and can really make the Durable feat shine - if you're a high level caster, having HD that give a minimum of 10 HPs each is really nice.
I also see this as encouraging a short "work day", as the casters (who already are the one who like short work-days) are now burning yet another daily resource. Sure it's to regain a different daily resource, but spending, say, 6 HD to regain one action's worth of high-level casting isn't pushing off the run-out-of-spells much while seriously impacting their other side.
Blue said:If I was looking for a rule like this, I'd:
* Give non-casters and half-casters things to spend HD on. (Advantage on an attack, etc.)
* I'd change the cost of spells so it wasn't linear.
* Cap the spell level you could regain. Definitely 5th would be the highest, just like Sorcerers and spell points.
Ahh...I did say "full Arcane casters"; so no Knight Errant, Bard, etc. Another thing I didn't think I had to mention was the whole kettle of fish that allowing Feats and Multiclassing could bring. But I don't use those options in my campaign, so it never entered my mind honestly.
Point 1...I only would allow FULL Arcane casters this option.
If I'm reading it right, it's already got a bit of balance built into it: casters refresh faster with a short rest, slower with long rest. As long as you don't allow serial short resting - resting 1 hr is a short rest, resting again is just a 2nd hour towards your long rest - you should be able to balance it with the right pacing. You might also want to limit short-rest recovery to slots expended since the last rest, so you can't have 'contrived' encounters to get back multiple slots of the same level.After a Short Rest all spellcasters refresh one spellcasting slot of each level per level up to their Proficiency Bonus (so someone with a PB of 6 refreshes one each of levels 1-6). After a Long Rest all casters refresh one spellcasting slot of each level they can cast.
How would you balance that?
Try thinking about it in the abstract.
So the Wizard went from 16 to 27 slots, the Paladin from 9 to 18 and the Warlock stays at 8. So you have massively increased the power of half casters, significantly increased the power of full casters,
Thanks for doing the analysis; I was aware it would be a considerable boost without balance. So how would you balance it? How about reducing the base casting slots?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.