Star Walrus was a game built to help you learn Haxe; it's funny and silly, but it can be more. At the end of each chapter, I suggested ways to tweak the game or expand on what you've learned, but the sky is the limit. To help with this, I've included the original art files used to make the elements in the game. They require Adobe Illustrator and Photoshop.
Let's go over a few possibilities.
Loading in predefined enemy patterns works for the quick pick up and play experience, but you could add more depth to the experience by expanding the game to take place over several predefined levels.
You've learned to work with both JSON and XML; either would be excellent to build out levels. To make your life a little easier, you could have levels composed of enemy patterns instead of making every single thing in every level unique. All you would need to do is add an id property to the patterns and reference those in an integer array.