Well development goes (something) like this:

1) Create a core mechanic [moving paddle, shooting bullets at enemies, dodging enemies]

2) Polish core mechanic [paddle width and speed, enemy speed, bullet speed, explosion radius?]

3) Create core loop [score goes up as enemies are destroyed, score goes down when enemies get through. Speed increases with score. Game over when player hit or score is zero]

4) Polish core loop [Game speed increase rate, scoring system possibly based on enemy type,  how forgiving of player mistakes]

5) Variations [different types of enemies, enemy formations and movement patterns, player powerups]

It's still at step #1 as shooting doesn't do anything.  You could decide if shooting is even part of the mechanic, if it is, make it do something and go to step #2.  If it's dropped it becomes a top-down endless runner and you go to step #2.

Good luck.