A modifier can be generated by combining different stats:
Example1: "+34% damage bonus to Fire spells vs Wild Pig in Mud during the Full Moon"
1. Damage applied to fire spells, to monster class, in specific terrain type, during specific moon cycle phase.

by making a modifier to depend on different stat(synergy stat):
Example2: "+30% to magic find percentage(based on dexterity multiplied by character level)"(the bonus is of course divided by hidden constant)
Other attributes can be relied on, restricting the modifier.
Example3: "reduces 40 magic damage(base on level of Mana Shield spell)"
Example4:"adds +130 fire damage(based on number of items in inventory, excluding potions)"
Example5:"reduced poison duration by 40%(based on number of potions in inventory)"
Example6:"mana regeneration increased by 43%(based on current damage/10 )"
Example7:"Adds 126 damage based on available mana percent(i.e. 20% of mana =20% of bonus)"

component based stats that affect skill component:
Example8:"aura range increased by 10% (based on Magic points stat)"
Example9:"Fireball movement speed increased by 22% (based on mana% remaining)"
Weather/environment can be used to theme modifiers:
Example10:"Bow-type weapons speed is 31% faster during the night"
Example11:"Reduces cooldown of Ice-based spells by 50% when on snow"
Skills can be directly altered.
example12:"Increases level of all Skills by 2 when hp is below 25%"
Example13:"Enemy resistance to Cold spells is reduced by 74%(based on distance from nearest river)"