I think having the spell be a ritual rather than a cantrip is a better idea. I had thought of that but I got stuck on the ten minute ritual casting time, but I suppose there's no reason a ritual couldn't have a longer casting time.
Sure. Or the ritual could last 8 hours, and over that time restore hps & HD. Heck, it could last 24 hrs, and allow spending HD after a short rest, too...
I mean, lets say a PC gets captured by a bad guy and they torture him. While DMing the situation I specifically say that the bad guy breaks the PC's hands, shattering his bones with a club. Then his party charges in and rescues him. They take an eight hour rest and, technically, all those broken bones are back to good!
Not even an 8 hr rest. A one hr rest and a single HD, or anything else that brings him up to 1 hp and he's functioning normally. HPs simply doesn't model breaking bones, severing limbs, or other severe injuries that bring lasting (even permanent) disability.
You can narrate the broken bones, and when the PC is back up to fighting trim and full hps, perhaps it's been splinted, or he has his weapon & shield tied on or something - and, through the usual heroic effort, he can fight and adventure at full effectiveness - at least, when it matters. The player can RP the injury in less critical situations, and you can narrate some results in terms of the injury, a failed check is because he's hampered by it, a '1' he's stopped short by the pain, an enemy's critical hits mercilessly hammers the injured limb, etc...
Or, you can add to the rules to cover it. Inflict a penalty on the PC - he can't use that broken hand, so no two-handed weapons, no casting while holding something else, etc - that takes time or more than just hp-restoration to remove. You could do that ad-hoc, or you could create a whole sub-system for it.
Also, yes, my idea would require a divine caster of some sort in the party. But hasn't D&D always kind of required that?
Not
always, no, depending on where you draw the line on 'divine.' If you consider the Druid 'nature' rather than 'divine,' you could get by with one instead of a cleric in 2e (when it got CLW at 1st level instead of 2nd). In 3e, a party might just scrape by with a non-divine Bard as their healer, or even a collection of WoCLW & a rouge with a high UMD, or just bushels of healing potions, once the costs became trivial, anyway. In 4e, 'Divine' was explicitly a Source, and all the Sources, even Martial, in the form of the Warlord, could restore hps (among other support functions originally done by the Cleric). In 5e, the Cleric is a divine healer (as is the Paladin), Druid debatably divine, and Bard a non-divine healer.
As it stands now, 5e /does/ 'need' a caster of some sort to provide in-combat healing, or PCs that drop stay down for the rest of the fight, and that can spiral towards a TPK. What your variation would do is extend that need to daily healing. IMHO, that's not a big impact: any competent party should have a support caster or few, already.
I think magical healing is what has always made fantasy RPG's work so much better than most other genres.
In-combat healing, yes. It needn't be from magic, though that's a ready recourse for FRPGs. For instance, Champions! combat works very well, and everyone more-or-less regenerates in D&D terms, via 'Recoveries' every Turn - but the combats are mostly about getting a KO, rather than a kill, so it's all 'STUN' instead of of hps...
