This is really a great idea! I honestly liked how you implemented the benefits and consequences of the power-ups, it was a clever touch.
The setting is beautiful, and I really like the game’s concept. The way you designed everything is truly creative.
If I may, I’d like to offer a few suggestions:
The power-ups could have more suggestive designs to help us understand what we’re about to collect.
More diverse behaviors for the slimes would be interesting as well, like some that emerge from the ground (with an indicator that they will do so), some that jump really high, or even some that fly, it would be fun.
Based on the idea of more behaviors for the slimes, there could also be challenging parts with rewarding paths that have more coins, like slimes flying in a line to a spot filled with coins.
We could use the coin score to buy an extra life if we happen to lose, but increasing the number of coins needed with each death and decreasing our score according to the respective cost.
A small scene transition throughout the game would also be interesting, I’m referring to the cycle of morning, afternoon, and night, which would add some visually appealing variety.
Congratulations on the game, it truly is a great job!