That could be something, but the biggest issue that comes to my mind is that some characters as a concept don't want to hurt others, and doing damage is just not a good option for them. But I can still see some good ideas come from this nonetheless.
So my idea that I can potentially build off of that is to have 2 sets of Hitpoints. One is the Hitpoints we all know and love, and the other are Mental Hitpoints, which exist as the exact same total of hitpoints. For example, Johnson has 75 Hitpoints. This means that Johnson has 75 Mental Hitpoints too.
When the normal Hitpoints go down, the Mental Hitpoints go down the same number. So when Johnson gets hit by a Longsword that deals 10 damage, Johnson now has 65 Hitpoints, and 65 Mental Hitpoints.
Then lets say that one of the foes that Johnson is fighting, a Cleric named Bob, casts Hold Person on Johnson. Johnson succeeds his Will Save, but he feels more mentally drained from the ordeal, taking, lets say, 12 Mental Damage, leaving Johnson at 65 Hitpoints, and 53 Mental Hitpoints. As Mental Hitpoints go down, Johnson becomes more and more vulnerable to the full brunt of whatever spells or abilities are thrown his way.
I know this needs a lot of tweaking to work, but thats why I'm here. I want something to work.