Hey people!
Thanks for the comments, I really appreciate them. As I had no intention of implementing a story mode I made it hard on purpose, as there's only the next highscore to reach.
I'm planning to do a sequel which features a nice story-mode, power-ups included. I really wanted to call my first attempt at using the flixel engine done.
@FreakFellah: The difficulty is in fact slowly rising and thus there are more and more enemies coming at you. I'll look into making it more wave-like and see what happens.