Right now the enemies all gravitate toward the center of the level, over time. Instead, for each computer-generated level, the game could select a different location at random and have the enemies gravitate towards it, instead. Their behavior would be much the same, but the player would no longer know that he can find all the monsters in the center if he waits long enough. This would make the game slightly more unpredictable, and eliminate the cheap tactic of standing to one side of the level and firing down the center line.
A more sophisticated approach could have multiple gathering points so that the monster wanders toward whichever is closest, and custom gathering points for the special levels. I don't know how difficult any of this is to code, though.