Thank you for the feedback!
As for the variation, I'm planning to add secondary weapon power-ups and I might add some new enemy types too. I initially planned to generate varying sizes of levels but then settled for the current relatively small size to keep the average level completion time short.
How far did you get? If you got to level 8, then you probably saw all the enemies there are.
The game is being developed on Linux too ;)
Thanks for pointing out the missing input methods. I had forgotten to fill any of the metadata for the game.