I like this idea, and actually already employ something similar in my own fights, but not so solid and rounded. I like the idea of having different "levels" of baddies, so that if you face say, a group of goblins, there will be slight variation in their defense, HP, strength, etc. Your explanation seems to be a more solid and reliable solution.
As far as the fear thing, I also employ a SAVE vs. WILL but my target number fluctuates. I take into account a couple of variables. For instance, if a group is surprise attacked and one in the group is instantly killed, they are much more subject to fear than if they had already been fighting the characters and one in the group falls. This could be as simple as creating a table of modifiers for the TN such as:
Surprise Attack: +2
Susceptible to fear: +3
Players quickly defeating enemies: +2
Players make an intimidation roll: +1 for every level of success
Just some ideas. My numbers aren't solid, just jotting something down some thoughts to get the ball rolling.