Choose something that interests you and something you want to use yourself if possible. If you can also try out a new language or some new technologies along the way, even better. If you build something with no expectations of making money, then everything is a bonus and you have had fun and maybe picked up some new skills. I would say for most side projects, the majority of the value is not gained from the revenue. Having said that, completing something and putting it out there is the most important part. Good luck!