Agree with Zappan, this is a great concept that will surely make a nice game. Some more suggestions: it's kinda frustrating when you dont have a "quit game" button somewhere, and I took sometime to guess that ESC did that. It's quite obvious, I agree, but some HUD elements would get the player more confortable. The physics simulation should be paused during design mode. Check Bad Piggies from Rovio, it has a nice design/play concept and might give you some nice ideas. Last thing: make the controllable muscles colored, this may give some nice visuals and get things more clear to the player.
Well, I see some potential in this idea, of course it is in a very early stage of development, but it may get to something good. Maybe you should focus first on some gameplay issues, like showing the player where exactly he is pointing the gun at (maybe a crosshair), making the zombie fight back somehow (no need for animations yet, but you should get some damage when he touches you), increase the difficulty of the waves, this kind of stuff. When the game feels more playable, then you get it pretty, with animations and graphics and proper hands. 3/5 because I like the idea of a wave-oriented zombie survival FPS with a destroyable stage. Suggestion: In the future, the waves should start sequentially, so new zombies would just arrive from somewhere and the stage woudn't reset. This could get interesting when the whole place is destroyed and you don't have much places to hide.
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