How about this: two warring nations are separated by a deep chasm with one narrow bridge and fortified towers on each side. Call the sides the Red side and the Blue side, if you like. Even better, the chasm is a Dealdly
Chasm of Non-Stupendousness, meaning that all heroic Feats attempted above or across the chasm will automatically fail.
Now the funny part is that a bunch of Red heroes have gotten stranded in the tower on the Blue side, and a bunch of Blue heroes are holed up in the Red tower. They can't get back to their home countries because they'll be killed by the guys in the other tower if they try to cross the bridge. Their only hope is to hold out long enough for their own country's army to rise up and take out the other tower. So while the Red heroes are under siege from the Blue army, the Blue heroes are also under siege from the Red army, and the army sizes get bigger as described in the earlier posts. Once one of the armies takes out the enemy heroes, they'll be able to safely rescue their own heroes back across the bridge.
How do you like that? Both players get to play heroes and armies, and it's much better consequences to motivate haste.