Haha that's a great question! I think I will write a devlog on that in detail at some point, but I can give a quick sparknotes beyond my description above (which basically summarizes that, it took a lot of iteration, brainstorming and pivoting to come up with what you see now).
At the very beginning, I was inspired by this video from Game Maker's Toolkit:
I saw Snakebird and was like "I love it, I wanna make that". After like 7 months of development, I was disappointed that the game could simply be completed and there's no remaining replay value. Around that time, I saw this video: Which talks about the concept of making a "home" within a game. The idea is that there should be something to do in the game that isn't the strict progression of the main quest. I thought about how endless puzzle games like Tetris and Puyo Puyo have this. So, I tried to blend them together with my game somehow. I failed to make Tetris interesting, but Puyo Puyo's mechanics were deeply compelling, so I rolled with it!tldr;
It's hard, and I tried a lot of things and took lots of different forms of inspiration until I found something that works.