This deserves to stay on Newgrounds, the graphics is very good. You could add some improvements:
1. Some kind of a background story, what it is all about;
2. Levels;
3. In relation to previous: upgrades of weapons and shields, in accordance: new, harder enemies on later stages;
4. Progress saving;
5. Pause button/option;
6. For me personally: also an alternate arrows movement, maybe I am a traditionalist, but I just hate this wasd scheme;
7. If the game would be expanded enough you could think about adding some medals.
I think that this is a recipe for a solid thing for Newgrounds. There is no need to improve graphics, because as I can see you definitely know how to do it and this is a strong point.
You could create - based on this one - some kind of a game with some levels, stages and upgrades bought with the usage of money earned by killing enemies and if you would like to add them - by completing quests (killing some enemies, maybe surviving without hits, doing something on a map or others).
The game has definitely a potential, so keep it up!