In most RPGs, health does not replenish between battles. You have to go to an Inn.
Getting ground down over a large number of battles, with enemies taking a small amount of HP at a time, is boring. A solution to the boredom problem is to have a smaller number of battles where the enemies do more damage. This now poses another gameplay problem: normal enemies will tear you apart unless you have healing items, so it is possible for part of the game to be impossible if you are one health potion short. The solution to that problem is to make it easy to reheal to full health between battles by making healing spells cheap and putting healing areas in dungeons. Some game developers threw up their hands and let your party automatically restore to full health between battles. We also have a conceptual change in that HP no longer represents fatigue. If you restore to full health all of the time, you are no longer getting ground down.
Let's split the middle.
First, we will make the combat attributes - HP, MP, Mental State, etc - the final result of a multi-stage process where one stage draws energy from the previous stage over game-time.
Core -> Reserve -> Immediate
- Your Core energy is a huge stockpile that represents all of your energy for the day.
- Your Reserve energy is refilled from your Core energy at a relatively low rate. That is basically its purpose, to prevent Core from being drained in a single encounter.
- Your Immediate energy is what you use in battle. You cast magic using MP. You swing a sledgehammer using physical energy. Your Immediate energy is refilled from your Reservoir at a relatively high rate. You can get tired by fighting too many things at once, but if you get away and rest a while, you can recover.
The result is a system where your HP will normally refill between battles, but will not be refilled all the way if you immediately run into a second battle on the next square.
Now let's extend the idea. You can fiddle with the transfer functions. Maybe they work slower as Core goes down. Maybe they work slower as the target reservoir fills up. Maybe there is an external variable that pushes down your maximum refill over time. The result now is a system where you might restore back to full health after twenty steps, but five battles later, going twenty steps may refill you back to 80% health. Ten battles latter, twenty steps may refill you back to 60% health. Your dungeon divers are now getting ground down over time, and there is a logical and gameable system behind it.
If you need energy RIGHT NOW, you could cast a spell to refill the Reserve by burning a significant chunk of Core. You might be healthy again but your health will not restore as quickly as the other party members. Suddenly there is a downside to casting those cheap healing spells.
Your spoony bard and spoiled princess may run low on Core energy and start falling over before the hardier blacksmith, ranger, and solider. The energetic attacker whose skills go all-out may run out of steam before the others. You might be encouraged to swap out the active members of your party to let the members with low Reserve recharge.
This system could work well in action RPGs or any RPG with a day/night cycle that tries to represent the passage of time.