Start with a mechanic you can test in a week
Every project begins with a mechanic, not a setting. A mechanic can be built, played, and judged in a short span, which means a bad direction gets caught before it consumes months. The prototype is deliberately ugly, and that is fine.
We are testing whether the core action is interesting without tutorials, art, or a story to carry it. If it is not fun at this stage, no amount of later polish will fix it. So the decision to continue happens early, on evidence rather than enthusiasm.