what i was thinking is 1-5 is critical fail. i.e character does to himself what he intended to do to the enemy, 6- 15 would basically be a normal thing, if its a physical attack close would do unarmed damage, if its a ranged attack it would fizzle, but not inform the enemy of whats going on, no harm done, and a 16-20 would be a critical hit
All ability rolls would be decided on a coin flip pass fail. No mods.
this is what i was thinking