So, I have some suggestions you could add to the engine.

1. 9 Polos
2. Bonus characters
3. Add pages to the credits just like the gallery. For example, there will be pages for the Dev Team, others, and so on. It would be cooler and better
4. Sun in background (I have a sprite file for something like this, if you want it, I'll give it to you)

That's all I could think of. If you add any of these, feel free to let me know.