The Octahedron

I think my dice mechanic will be a simple d8 + modifiers, with the main modifiers being the relevant attribute. In contested rolls, the defender automatically gets a 4 instead of rolling, and ties go to the defender.

This allows a decent range, and means that on an average difficulty (where you need a roll of 4 to succeed), +2 (remember, this means a doubling of your attribute's value) halves your chance of failure, and -2 (halving your attribute's value) halves your chance of success.

Without adding some kind of way to get extra bonuses or something, though, this does mean that a character can, for instance, never hit someone in a fight that is four times as skilled as him. While that kind of makes sense, I wonder if there should be some sort of low probability of automatic success.

Or some kind of 'luck point' resource players can spend to add to their rolls or something, for the one-time-only dramatic success type events.