Thank you for your feedback! From the Rockbox games I've played, I found this one the most memorable. There is a strange kind of satisfaction from getting a chain reaction from a well-timed initial action. Regarding your points:
- In the original version I've played, I think level 12 was the last one, allowing people to actually see the winning screen. I've considered to remove the extra-hard levels, but your suggestion of restarting the same levels may also work. Alternatively, a level select screen for those previously completed (this is a design I've seen in visual novels and it makes sense to me, although I've rarely made use of the feature).
- A visualization of the remaining counter could be cool. I thought of adding some unobtrusive status text or pulse or whatever (it's a countdown of 3 seconds since the last reaction). Animating the balls themselves definitely requires more thought, I found it difficult enough to animate the ball movement/enlargening/shrinking.
- Sounds would be cool, but doing this in elisp is tough. If it's just a single sound, it's doable with a mplayer/mpv subprocess and pipe, I've done that in chip8.el. The downside is the extra setup required from the player and it was difficult enough to ensure the game works for other people. Maybe better suited for a more general-purpose lisp? (I'm partial to CHICKEN)