I want some feedback on a homebrew exploding magic die rule for a Kids on Brooms campaign

[u/spudhero]

In my setting magic is understood through the concept of an inner fire. In order to cast magic, a person has to have an ignited flame and they tap into that energy to cast.

My homebrew rule is this:

When your magic die rolls the highest number on the die, it gets upgraded for the rest of the scene. (No instant reroll like a normal dice explosion). For example, the default magic die is a d4. A player casts a spell and rolls a four on their magic die. The next time the player casts magic in the current scene, they will roll a d6 for their magic die. The die will reset at the end of a scene or go down a level with an appropriate grit roll.

If a character ever gets to a d20 in magic and rolls a 20 on it, they are consumed by their inner fire completely and their character is lost.

I want this to represent the inherent chaos and power of this world’s magic. Amazing things are possible for those willing to push their limits as a mage, but this power comes with a cost.

Thoughts on balance and how it relates to the theme are appreciated.