Sounds cool. I can imagine how the stars/coin idea gives incentive to the player to take the more challenging route, effectively increasing screen-time, which is great. I also see that you're also using Game Maker Studio. For the inventory, some kind of resizable data structure would be my first instinct. I haven't looked into the data-saving functions yet, but the new GMS 2 IDE is pretty slick so far.
Looking forward to more!