Currently this implementation of D20 isnt very fun...
The point of the die is the thrill of rolling a 20 and the horror of rolling a 1. Each has a 5% chance of coming. Taking it a step further, and actually put different special effects on 17, 18, and 19, as well as the 1 and the 20 would make it so much more interesring.
That means that there’s 25% chance of something extra special happening on any given action–5% bad and 20% good.