There really needs to be three basic default reactions added.
1. Melee attack- If an enemy enters melee range while this reaction is set, your character attacks with a melee attack
2. Ranged- If an enemy enters range and LoS of my character's ranged weapon, my character will use a ranged attack
3. Offensive Spell- If an enemy enters range and LoS of my character's chosen damaging spell, my character will cast that spell at the enemy