When I start designing my game, I usually have a vague idea or game mechanic I want to play with. I then make a super basic game to playtest it with others and go from there, deciding what makes the game good and what updates would make it better.
My inspirations in the past were parkour, other games, and everyday culture.
You can find inspiration from real life or fiction. Find what you love and run with it.
However, you never know if your game idea is good until you playtest it, so do it QUICK and ROUGH.