Thanks for the feedback!
I'll probably add extra lives as power ups that can be obtained having geting high score, it will probably make longer and more intense sessions.
A tip to keep your combo going is not to kill the enemy ships as soon as you see them, try keeping them on the game and farm their bullets (destroying bullets keeps the combo going too), but don't let a lot of enemies on screen or you will be in trouble ;).
I'm happy that someone mentions the music too, I didn't give it too much time but still wanted to have something of my own to play on the background, maybe I'll make more sections and reduce the repetitions of each section in order to have more diversity.