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Cool game. I really like the concept of it, but i have a few suggestions.

  Having alternative areas that spawn other then just more TUBE would make it a much more interesting environment. 

 Items you can find would be nice. New weapons and gear.

Different enemies would be good. If you want just one type of enemy i highly suggest more of them with less HP. Fighting 3-5 at once that only take 2-3 shots would be more exciting.