Looking at the game from a systematic point of view;
- I would want to add collision to the environment + enemies first through the OnCollisionEnter (adding explosion particle effect on collision) method.
- I would then potentially look at adding a raycast so when you shoot enemies with the ship they die through something like destroygameobject or coroutine method + trigger explosion particle.
- Add ability for enemies to shoot at us through instantiating a similar laser
- Add health function for when we get hit
- Perhaps add movement to enemies so they move across the screen
- Add a score card for counting points