[Macunaima]

Here is the rule I would use:

Units that do not move or fire in their turn get a reaction fire marker. After the enemy’s first movement phase but before his combat phase, units marked with the reaction fire marker can conduct a combat phase of their own.

Reaction fire markers are not removed if the unit participates in overrun combat.

This, to me, would be a simple way to address the problem mentioned in the beginning of this thread.

Scenarios would have to be tweaked to take this rule into consideration. An increase in the attacking force of 20%, perhaps?