So I recently watched a bit of Grim Dawn, a game I havent played myself, and the constitution mechanic intrigued me. It is displayed as a separate “health bar” on top of your normal health bar, that effectively gets eaten away at by enemies in tandem with the regular health during prolongd combat.
The basics of it is this:
In combat you loose health, but when you disengage your enemies the regular health of the player is rapidly refilled without using potions, instead spending constitution in the process.
If you loose half your health the constitution bar will also be down to half and remain there after combat until refilled with a secondary mechanic.
(for D4 the ammount of constitution used to heal the player and the way to replenish constitution are issues I leave to the devs)
https://grimdawn.fandom.com/wiki/Game_Mechanics#Constitution
To me it seems like a fine mechanic to help keep the action flowing while ensuring that the player can stay in the fray for longer while still having to actively manage their health, as it allows the players health to fluctuate heavily during combat without needing to resort to potion spamming (which we all know wont be a thing in D4).
This mechanic would work especially well with tanky melee builds and would make it so that the tank wouldn’t necessarily need to focus so heavily on stuns, as they could disengage the enemy and reposition as an alternative means to heal, saving the potion to when in a more dire situation. As you know this tactic could of course not go on forever as the contitution bar is diminished when the health is restored.
I personally don’t see the point of this mechanic on builds other than tanky ones. Flavour wise maybe it could be a shared Barbarian and Druid trait, that could be unlocked far down the passive node roots of the skill tree. So if you want to make that tanky battle mage of your dreams you would need to get ahold of the unique item that grants the effect 