I forget what war it was but in one of the two World Wars I remember hearing about a guy who was stuck in a tower with a sniper and like 300 bullets and he fended off hundreds of German soldiers by himself.
Imagine how cool a challenge mode based on war stories like that could be? Re-creating what heroes(and villains) of the past allegedly achieved.
Could make for a good co-op mode too, 4 men in a town defending against waves of enemy troops.
A nice extra and heck, could be a little educational.