I have an interesting thought on this that could open up some varience.
What if all the monsters were leved 1-4 and the only difference between a level 4 monster and a level 1 is that the level 4 gets 4 passive purple to hit? The level 3 gets 3 passive purple when attempting to hit and so on.
And the heroes are a level = to their rank. So when the hit rank 2 they get 2 purple passive defense, rank 3, 3 purple, and so on.
I think it allows for making 4 different kinds of each monster
Mook
Standard
Veteran
Elite
And it's just as uping the purple dice for each version to hit them, while the PCs get better defense over ranks.