Your concept - avoid'em'up - is original. I don't remember playing such a game. And the gameplay is rather enjoyable.
At the end of each delivering mission, the player's ship may have to dock to a slightly moving space station. This new game phase would add to the interest of the game. And then you would have the fundation for a 2D arcade version of Elite !
Also, consider changing the shape of your power-ups to a square, for example, so the player will be less likely to mistake them with enemies.
Ideally, improve all your graphics. Purple enemies may become meteors, cyan ones comets, the chasing ones saucers...
Thanks for your feedback!
Ah, Elite. I remember the docking episodes - it was very hard, and very easy to crash!

It's probably a good idea to change the shape of the power ups. For color-blind people it's probably impossible to see the difference between enemies and powerups. I had not really thought about that...
It would be awesome with general graphics improvements, but there is not very much space left unfortunately (to many enemy types). But I might be able to prioritize away one of the enemies, and maybe save some bytes here and there ...