Play other indie games similiar to the genre you want to make and while doing so take notes on how the mechanic works; what aspects people like most and least by also checking the reviews
Then try to make something similar by adding your own style and fixing the weak spots while still maintaining your own identity
While thinking of that also try to brainstorm new ideas or spin, maybe two similar games have different mechanics that would create a novel experience if mixed together, or perhaps you might think of something new entirely, idk
Afaik this is usually how people create quality ideas for games, they get inspired by projects they like and create their own spin of it