I'd like it if it were directional flanking based on what region you're coming from.
Let's say the defenders are being attacked by a region on their left, and a region on their right at the same time. The region on the right is closer, in terms of miles, so that attacking army is facing the defenders head on.
The region on the left arrives after the first turn, and gains a 25% bonus to defending units ALREADY ENGAGED with the other army.
The defenders scouted appropriately, and decided to have a reserve of infantry in the back, wall setting archers in the middle. These infantry engage the flankers, negating their bonus.
The defending archers are set to skirmish, allowing them to attack either army depending on eligibility; which unit is closest to them, in range, and not engaged in melee- to avoid friendly fire...
An idea at least, but probably an idea from he'll in coding perspective.