There's not much to do to get a nice gameplay:
- the controls are too static at the moment: accelerate them while holding a key
- add some basic alien movement, collision detection seems to be easy enough with your system
- increase the random value of the cave borders while playing
- add bigger aliens which shoot as well
- and so on
- not necessary but nice features, easily added: changing colours of the pixels restricted to the entities by their skills, mines, different weapons,...

Probably, you are more intersted in features which are requested in the contest: paint the caves borders, add a structure depending the shape of its border, generate some background effects...