Probably the best thing I have come to understand in building side projects is that you need to get it out in front of people as soon as possible. After that, just get feedback, use analytics, and iterate quickly to fix issues.
Even more important, go look for existing problems, whether it be in online forums or talking to people in person. If you making a unicorn detector and you spend several years with your nose down, its a rough lesson to learn that no one wants a unicorn detector.