Good game, i like shoot em up !
The ships are good in design i find
I would prefer lot more dynamic on screen : effects , explosions,
more bullets etc ...
You could use better shaders, the ships are dark, like level, you could use some more shiny metallic look ? Specular shader ?
Perhaps some fullscreen bloom could be cool also ?
What collision system do you use between player ship and ennemy ships ? and for player bullets ?
Keep it up !