oooh rain world is fantastic. i agree, something to collect would be great! i was thinking the wave could wash things up that you have to get before it comes back c:
one other idea for the sake of progression I came up with a while ago was the wave washing up a bottle of some kind of potion that allowed you to 'level up'. It made the game harder and easier at the same time. Though critiquing my idea I could have made the good side of leveling up through washed up items that could help with progression. Dang maybe I should finish this project 😅