I am not an adept of variantism, as many may have noticed over here.
But, thinking of the marked preference by some players for large monsters, I am asking myself if smaller ones could be given a really good defense dice pool (such as a black + a gray dice).
One imagines that the defense dice reflect armour/protection, but they easily can be a "design for effect" for being small and swift.
There could be an advantage when attacked at a distance (an additional die), to reflect their smallness.
When attacked by an adjacent hero, they could have a "spring about" capacity (tested like a heroe's attribute, with a fixed number) which could allow them to move one square further than the attack (which still would be counted as having taken place). They could thus escape blast effects in some occasions...
Imagine zombies and goblins with such features.
They would be much more used in open groups.
There would much less testimonies of heroes wiping a whole monster group in one blow/turn...
Heroes would really come to hate those pesky, tricky little beasts !
In any case, such features could simply be given within a scenario special rule.