Proper enemy spawning.
More enemy types.
Variable difficulty (bullet hell mode?).
	AND/OR:
Multiple 'pre-determined' levels (enemies are always in the same pattern in each specific level).
	-this could be achieved by using the same random seed each time a particular level is played or programmed levels using a similar system to the enemy types one
Power-ups and abilities? e.g shield, more bullets in multiple directions, etc.
Better system for finding empty space in array for new enemies and bullets.
Music and sounds.
