Make Toughness Retroactively Change Maximum Hit Points
Currently, your toughness score only affects hit points gained when leveling up. This means that when your toughness score is modified, it will only have an effect on your hit points when you gain more levels. This is a bad thing because it makes temporary toughness changes (e.g. potions of good health, or the sickly status) largely irrelevant in combat. It also incentivizes tedious behavior like equipping and consuming toughness boosting items right before leveling up. I suggest having your maximum hit points modified by some amount per level determined by your current toughness score.