This idea was awesome enough to make me finally get around to registering. It sounds great, but I was thinking it would be even better if it included some tower-defense-esque elements, such as the enemies gradually becoming stronger, your arsenal gradually becoming larger and more awesome, etc. Actually, I think I'd really like it if it was some sort of era-progressing game; you start with stone-age weapons and enemies, and it gradually crawls forward in time, giving you better and better weapons to combat the improving enemies. It's not a story, but it would still keep things interesting through change. Maybe, for example, you start in a cave with a sharp rock, you fight your way to the mouth of the cave, you find a knife or a slingshot, the enemies change slightly, repeat ad infinitum. The enemies would always funnel you towards the next weapon. The collection of said weapon might unlock the option to start a new game from that point, or with that weapon at an earlier point, or with an earlier weapon at that point. Multiplayer would just be the ability to jump seamlessly in and out of a friend's game and fight together or compete to kill the most enemies.
Sorry if that was a little incoherent...