No wonder I was confused.
You mention God of War in the title
and then go off on Gears of War 2?
haha my bad dude we can throw Kratos in a wave somewhere as a boss.
As far as it being just like playing the game that's not a bad point. you do pretty much do that already, but in this there are no trips to town, no new items no switching gear, no potions/consumables its just you going until you complete all waves or you die. I guess you would have to address what happens if your buddy dies during a wave and you don't? do they get rezzed at the start of the next wave? or maybe you get a scroll of revive for every x amount of waves you survive.
You could also make it a competition. you could set up teams to see who was the best of the best. Give out stats to players to see who is better. Who took the least amount of damage, who did the most damage, who killed the most monsters, who stayed alive the most. You could even make a ladder if you wanted, have quickest clear times, or highest waves without dieing.