Something that could help make the game a bit more fun (for people with fast PCs atleast) and more marketable would be more eye candy.
-More textures
-Dynamic lighting
-Weather
-Particle effects
-Making the bridge and train look more reflective
-Multiple environments (arctic, forest, tropical, etc)
-Pretty water (is pixel shading out of the question?)
-Hardware T&L to keep frames up
Of course many of these are out of the question, but small steps in a more graphical direction would be very cool.