Dissimilar assets .. very important for tactical play:

ANOTHER IDEA in this manner: besides weapons that deal “piercing”, “slashing”, or “blunt”, and armors that resist in different measures, HOW ABOUT
Have it in the rules that you can vary your attack to have jiggle the damage-type around a little bit. (I got this idea from Order of the Stick #216)

Varied attacks to try and better your effectiveness against the opponent, even in the face of resistance. This would incur a penalty and would be roleplayed as the character trying really hard to MAKE his weapon hit appropriately. EG skeletons are immune to, say, piercing, and so a rapier would do no damage normally. However, by using a rapier in a normally far less efficient way, he can hit the skeleton for non-piercing damage. Similarly, a rock monster immune to blunt damage may be hit by a dwarven hammer if the wielder aims for fissures.

The GM should be able to veto this ability if inappropriate.