And if you choose wrong, you'll find out soon enough. Personally, I had a lot of projects I couldn't stop thinking about, but when I finally started working on them, I quickly realized it's a bad idea. The usual reasons were: a) I already found it done by someone else in a form that satisfies my needs, b) it became obvious that either the process or the outcome won't be as satisfying as I thought it would be, or c) after scoping it out, I realized I'm not in a position to invest the required amount of time and effort.

Abandoning a side project early is not a bad deal. Your research and thoughts put into it stay with you forever, and it often happens that you'll get secondary value from them on some other endeavor.