What I've always though would be awesome is a zombie invasion sort of thing. A lot like your idea but you have to defend a castle. There is a mod for this that could be used as a reference? it uses a block that all the mobs try and destroy.
Video of the mod: ( )
Obviously you wouldn't be able to have all the extra shizz like the traps and swords and stuff. Or even have the mobs break down blocks?