Unified Theorem of the Dice

Right, so I've been talking lately with some of the others out there; you know who you are. We've decided that we're going to come up with an alternative to the blatant d20 ripoff we've been partially subsisting on so far.

My current thought is that a basic attack action would run like this:
Attacker states in OOC thread that he is making attack, then rolls 3d6.
Defender states in OOC how he defends from said attack, rolls a d20 in response.
GM interprets the results, informs the players, and they post their writeups in the actual story thread.

A lot of this is predicated on the GM maintaining sovereign control should he/she choose; the GM's interpretation may include modifiers AT THEIR DISCRETION. Say it's dark. The GM therefore assigns a minus two penalty to the attacker's roll on account of it being dark as all get out. And so on.

Sounds reasonable. Just to be clear however, if I were the attacker, I'd be rolling for my attack(s) only and the GM or affected player would roll the defense die on a separate post, right?

Ostensibly yes, to prevent accusations as much as possible, though one could arguably streamline the attack and defense rolls into one post.