I wish all units (normal producible units, heroes, nobles and the Sov) has one or more personality traits. Monsters have personality when applicable.
Personality trait should pay important roles in a few key areas of the game, namely Morale, Marriage, and Diplomacy. In this context, Mood is similar to personality. For example, a hell hound will be literally 'permanently' angry. I use these terms interchangeably here.
Personality is a big deal when there is any negotiation happening btn empires (e.g. tech trade), in any recruitment scenario, or considering turning coat when being bribed. Whether a deal happen or not, or at what cost, it will be so much affected by the personality interactions of the 2 parties involved.
Personality also affects how player compose its stack, due to its effects on Morale. Some unit combo just don't match due to their personality difference. A very rough example can be a Angel Demon stack will have extremely low morale. A morale of a hero can drastically different when they met setbacks. Some heroic character may even has a higher morale after a setback, but most other personality won't. Personality will have actual effect to statistics, dice roll and especially (troop) morale, when an appropriate condition is met. Your hero will have really elevated morale, when he attempt to rescue his sweetheart under a siege.
Personality (and Profession, Talent and Weakness) compatibility makes or breaks a marriage. It affects whether a marriage proposal will be green lighted.
Personality should be injected into the "Data Driven Dialogue" system SD has already put in place. The language, the wording
will be different depending who is speaking or spoken to. This may be optional (or mod material) as I can see that a lot of extra writing will be needed. But it sure will spice up the RPG aspect of this game.
Personality should be an important statistic for all units. Generally, a player will only an empire composing units mostly compatible with each other. For example, an Fallen player will have lots of cruel, chaotic, erratic units, but they are all somewhat different when it is randomly generated.
As a statistic, most of personalities a unit have will be hidden from player at turn 0. It will be revealed bit by bit, maybe by spells or dialogue etc. Probably the only exception will be monsters, a Lich will always have a "loathe life" personality.
This idea can also be considered as a continuation of the idea The ascend of a Unit to Sovereign & Leadership Value. I wish almost all units will have Personality on top of Profession, Talent and Weakness.