Definitely an interesting idea! As mentioned by others, a difficulty curve would make the gameplay more replayable.
What might be another interesting touch but is probably pretty hard to do, is have the background have more dynamic growth. Instead of just shifting the mask back and forth, it would be cool if the newly "created" ground starts barren and has to "grow" (buildings erected and different flowers blooming). That would create something interesting to watch because the sections that last longer keep growing and doing interesting things. Then you could even go so far as to have the things done in those sections be tied to some events. For example, if the bebes start building deforesting materials suddenly they'll start taking over the grassland. The only way to stop it is to help the grassland over-power that section destroying the deforesting materials.
I don't know, those were just spitballs. Congrats on your submission!