• Definitely a setting that appeals to my sensibilities.
• Could have done more to sell the thematic flavor, especially for the Communist aspects.
  • Would have benefited from sample locations and/or enemies
  • Suggested weapons or vehicles would have been helpful to sell the motif.
• Another possibility: specific mechanics for using vehicles and equipment, especially for combat situations. 

Doesn't need to be overly complex. Just enough to distinguish your personal vision from the basic Tunnel Goons rules.