I'd like the "allow fire/hold fire" control to have three states: Free fire, Cautious Fire, Hold Fire; where "Cautious Fire" means to hold fire if there is any chance of hitting a colonist.
I was thinking about this. The game uses toggle on/toggle off for lots of the controls, so it might be best keeping the "hold fire"/"Allow Firing" toggle, which the game currently uses, as a separate toggle from a "cautious fire" or "safe firing" toggle.
So when switching a colonist to manual control, "Allow Firing" would be set to on, as it is now, and "cautious"/"safe" firing would also be set to on. Colonists would then automatically shoot at hostiles, while not shooting at friendlies. They would automatically switch to shooting at a different hostile in such circumstances, if a target was available. This "Cautious"/"safe" firing toggle could be turned off, if a player desired to for some reason. The "Allow Firing" would be a separate toggle to completely turn off all firing until switched back on again.
And I'd agree that it would make things interesting if those who had the "Trigger Happy" trait ignored this, and perhaps had the "cautious"/"safe" firing toggle greyed out or missing.
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***NEW POST***:
If the idea of a "cautious" or "safe" firing toggle isn't sexy enough, it could always be turned on its head to become an "Indiscriminate Fire" toggle! With this toggled on, colonists would open fire on hostile targets without consideration for the location of friendly pawns (so the same as the game is now). The "cautious"/"safe" firing mode would therefore just be a case of not having the "Indiscriminate Fire" toggle set to on, and its default setting should be off.