Definetely some sort of Zombified Town stage, but as for something more original, I've always thouht it would be cool for one of the characters to actually have to go to Hell, similar to the Abyss in DoS, but you should actually have to fight Satan. Other than that, An armory where there are just tons of weapons and equipment for the taking would be nice. Then, the boss would be a ghost that possesses the weapons and you have to fight them.