Defense vs Offense

[FM WarB]

In other words, if you are in a full strength fortification, then any direct fire out of it would have twice the probably to disrupt, unless the adjecent enemy unit (target) is also in a full strength fortification.

Excellent idea. This represents attackers "hitting the dirt," which causes a unit reduced firepower, slower movement, less cohesion. It's an improvement over the old "pin" combat result which was known to stop attacks dead.
Something like this might be ported to Nappy and ACW games where the offensive advantage is even greater than PzC.