I'm ambivalent about hit dice being used as a resource to power more things.
I agree with
@Kobold Avenger and
@codo that it would fit the fiction of expending life force or pushing beyond normal limits to do something, which is cool, but I don't think that kind of feature would work very well with the current implementation of hit dice and resting.
In my mind, the biggest issue with hit dice is the way they fit into the resting rules. They have no value during long rests, since long rests fully recover HP, and can only be spent on short rests, which most classes don't much care about and a lot of groups don't take very often. As a result, they're a mechanic that, depending on party composition and playstyle, is pretty forgettable, "vestigial" as others have said. That means that new functionality for hit dice is gonna be inherently kludgy--and will also vary a lot in usefulness by party composition and playstyle.
Ideally, I think, PCs should be able to spend hit dice to recover HP whenever they're out of combat to keep their HP topped off (which necessitates you to think of HP as luck points or
not getting hit points, but no need to rehash that debate). And, since that would functionally eliminate short rests for several classes, it would be best if paired with short rest classes transitioning to proficiency times/day, at will, and/or encounter features (a direction WotC
sometimes seems to be moving toward anyway).
If those changes--or some other substantial revision of the resting rules--were made, then I think it would be cool to give hit dice further functionality.
I am not a big fan. My biggest issue with it, is that making hit dice a pool of dice you can spend for more power, makes have a dedicated healer character necessary. One of the best things about 5E is a party not having to worry about who has to play the cleric. [...]
Ya, any new set of hit dice expenditure options would need to
mostly avoid replacing their HP recovery functionality, and definitely shouldn't obsolete it. I think the best alternate way to expend hit dice would be the recovery or enhancement of class features that are defensive in nature, with a bespoke hit die cost specified by the ability, i.e. a fighter below half HP could roll X hit dice when using second wind or a monk with no Ki left could expend a hit die to use patient defense.
Hit Dice were pretty clearly intended to do more than they did in 5E, but at this point, I suspect we're stuck with them being a weird little vestigial subsystem. I'm not sure you should be able to spend them to get more offensive punch though. [...]
Agreed on both points. I would want to mostly avoid having hit dice deal be used to power features that deal damage, raise DC, improve attack rolls, etc.
[...] Personally I'd really like to see most levelled healing spells give you an opportunity to spend 1 or more HD in addition to the healing they do, at a minimum. [...]
I like the elegance of that idea. Though, depending on the group, it could lead to other players expecting PCs with leveled healing spells to share their hit dice, which might not be fun.
I'd prefer to see them gone from the game entirely as they kind of seem like an inelegant solution to the self-created problem of trying to balance adventuring around 6-8 encounters per day. But if they are going to stick around, yeah, there's a lot more you could do with them.
I tend to rate game mechanics by how intuitive they are to new players. For instance, most kids get armour class and hit points right away. To hit rolls vs. damage dice causes a bit of confusion, but not for long. Spell slots vs. cantrips similarly takes some explaining but they mostly pick up the idea before long. Action vs. bonus action vs. reaction takes a long time to sink in. And hit dice are really hard to explain and for beginners to remember, because they don't really make sense. Mostly I just tell them that it's a thing they can do during a short rest and not to worry about the logic. Hit dice are a weird, clunky resource and I think WotC know it.
This is the biggest problem, for sure. Hit dice are kludgey. And I'm not sure if my idea above would address their counterintuitiveness, or just make them more kludgey. But I think the rest rules would definitely need to change to fix their kludginess.