Exploding Critical Damage

Whenever you roll a critical hit with an attack roll, you get to roll extra dice for the attack's damage against the target, and when you use exploding critical dice, it can exceed extraordinarily high amounts of damage.

When you score a critical hit, you get to roll all of the attack's damage dice twice and add them together. Whenever you roll max damage on a damage dice with your attack, you can reroll the dice and add sequential results to the total damage. Continue re-rolling damage whenever you roll max damage with the dice, and when you finally stop rolling maximum rolls, add all of the results to the total damage of the critical hit. If the attack involves other damage dice, such as from the rogue's Sneak Attack feature, you roll those dice twice and reroll those maximum dice result as well.

PC-Only Rule

5e's System isn't designed to be as wild and swingy with the potential of critical hits or the overall damage-per-turn potential.

The concept of Exploding Die for critical damage comes from Warhammer Fantasy Roleplay & the Open Legend System. In their systems, such bombastic appeal is very fun and has great potential for generating memorable moments, hence why this idea is going to be put to use in 5e, for better and worse.

Monster Design as opposed to Character Design or mechanics are completely different, as Monster Design is fully within the hands and ability of morphing by the will of the DM. Characters however, are much more built like glass cannons in comparison, and there's never no telling how insanely dangerous Explosive Critical Damage works for Monsters with different damage dice calculation.

Image Source

One Punch Man by Murata (Chapter 106). I don't own the rights to this image.