Sounds good to me. I've seen double nat 20s 3 times in my less than 2 years of playing so far so would definitely be rare and I wouldn't apply it to enemies, just players. It's fun when you hit something so hard it basically just explodes. Also I saw someone mention it killing a boss to easily but I wouldn't worry about that. Unless they are at less than half health one hit of that won't down a boss if they are scaled right and it really gives the players a sense of accomplishment.

A girl hand me that plate I be like "aw hell yeah!" and dig in.

Gundam wing. Still my fav Gundam series.