I'm sorry to bump this old thread, but while revamping my website (http://xarthor.wordpress.com) I took a look at Pappenheimer's suggestions and the game itself.
Well the result was that I added the following stuff:
# increasing the random value of the cave border during play. Now the better you are the harder it gets
# Collision detection between bullets and the cave border
# Collision Avoidance between enemies and cave border
# Small Game Menu (Start, Exit)

You can read more about Alien's Cave here:
http://xarthor.wordpress.com/games/aliens-cave/

And here is the download link to the new version:
http://www.file-upload.net/download-1847658/AliensCave.zip.html