Effect
Amplification as a stat would be dedicated to improving the special attributes of a magic. This would apply to things like DoT, Puddles, Effect Timers, etc. This stat would be dedicated for either supportive roles or to give an extra hand for more niche playstyles.
Examples
Blindness from Light becomes easier to apply, making the effect more useful in both 1v1 and team scenarios. This could help someone to use it as a supportive magic, blinding enemies before either following up with a different, more powerful magic or to take pressure off of teammates.
Blindness from sources such as Snow or Sand would last longer and their damage boosting combos be (slightly) more potent.
Debris, Puddles, and Clouds could gain extra scaling with the users power and level, allowing for an area denial playstyle on magics that utilize them.
Magics with DoTs such as Fire, Acid, and Poison would gain damage and/or length, depending on the magic.
Wind would be essentially unchanged with the rework, as it’s innate effect would be boosted rather than being spread out to every other magic.
Magics that utilize multiple of these (For example, Magma with DoT and puddles) wouod have their effects halved on each (or perhaps could scale differently per magic, with say Magma being 75/25 between DoT and Puddles growth while Acid would be 50/50)
‎
While the exact numbers and nitty gritty details would obviously need to be ironed out, I think that overall it would allow for better customization of ones playstyle, and let an otherwise near useless stat have a purpose.
From @Alv : Amplification could improve the DoT of weapons as well as magics.
From @pika : Amplification can increase the amount of time it takes for effects to be cleared by charging.
Edit 1: Forgot to elaborate on DoTs for some reason and also made effects being less potent on magics that fit into multiple of these categories into their own paragraph
Edit 2: Added ideas from replies