In most classic Table top and video game RPG, HP and MP could be seen as the attrition of your expedition. The more you progress, the more points you lose and eventually, you are going to pull back. Since in many RPG, MP can be converted as HP with healing spells, your total attrition is equal to approximately:
HP + ( nb_of_times_healed X healing_spell_strength )
That has the consequence in some RPG of using MP mostly for healing and magic users most of the time hold back their spells for later in case something worst shows up. But it's hard to judge the encounters you are going to get later in the expedition unless you are going down to fight a boss.
So I had an idea to put attrition as a separate concept, and prevent players from holding back their spells since I want magic to be used intensively. The idea is simple:
At the end of combat, all HP and MP are recovered to their maximum.
The amount of HP and MP would be a bit smaller, since you only need to survive a battle, not the entire dungeon. Now that has the consequence of having infinite HP, therefore no attrition. So my idea is that certain stronger attacks(or added effects) would cripple your character which would reduce his max HP for the entire expedition. Draining monsters, could reduce max MP. So you would get a form of attrition over the course of the expedition, but you do not need to hold yourself during battle. You can use your whole MP for the duration of the battle since you know all the monsters you need to fight against.
If there is party spells that last for the duration of the expedition, it would reduce your max MP while the spell is active. So it would reduce the potential you have in each battle. This mechanic would also avoid the bug in Wizardry, where you can skip going to the inn by sending your spell casters at the inn and healing yourself with spells in the dungeon. As for items like healing potions, either they are usable in battles as healing spells, or either they can recover the max HP to reduce the attrition.
How do you like the idea? Did I miss any problem?
I have "darkest dungeon", I just did not have the time to play it yet. I guess I'll to explore that game too.
In my case, HP could be seen as stamina, so when you get hit, they are not serious injuries and you can recover easily. Maybe if you lose all your HP you do not die and you could come back at the end of the battle if the rest of the party wins. But my objective if to make characters die often, so I might not use this route.
Else, I was thinking to allow MP recovery in other ways, like draining monsters MP, meditating, etc. But for meditating, it implies that after battle you would meditate and recover everything.
I cannot just recover MP and not HP, because player will auto heal with their spells. So HP implicitly heals too.
Else maybe setting up a camp to rest could auto recover HP/MP. Maybe you can only setup camp is specific areas to avoid camping after each battle. This is an in between solution (You have temporary attrition and permanent attrition). You do not recover every battle, but you can recover often. Maybe camping grounds can be used once per expedition or you need a special item to camp. So you must plan your camping. Or maybe like souls game, camping makes some monsters returns.