You are onto something
This is very good work as an engine for a game in development. You should really do more work on this as it has the potential to be quite a nice little shooter.
The graphics, sound and animation are all fine. However, the collison detection is a little random. I flew straight through an asteroid without dying! This needs to be fixed. A lot of work also needs to be done on level design. Nobody minds arcade games being repetetive to a point, but the pace should be a little better judged. The action takes a long while to get going, with few enemies on the screen at once and little challenge. You should consider making the enemy ships fire bullets at you.
For inspiration I think you should really look at the design of games such as R-Type 3, Ikaruga and Gradius 3. Nobody expects you to make a game as good as these titles, but they will give you insight if you pay attention to the way the levels are structured.
You have a lot of potential