I also work full-time in software development. I had this problem of deciding on what idea to work on. If you have one idea that solves a problem you have, scratching your own itch is good. If not, I would look for an idea that solves a problem you know people have. If you are still not sure, signup for something like OppsDaily.com newsletter, it sends out a daily email to subscribers of a problem someone is willing to pay to be solved.

For me, I chose to scratch my own itch. I work on a food side project called https://bestfoodnearme.com but my time is extremely limited to a few hours a week. I keep a sketch pad to write down my ideas for the site. If I have ideas related to other projects, I write them down in a more general pad apart from the sketchpad that is focused only on the food site.

I put constraints on which features and ideas I need to work on. If there is a bug in the basic functionality of a project, I tend to work on that first. For new features, I will only consider them if I get feedback from at least 3 different unrelated people that mention the same feature to me.

I always plan out the night before which tasks I will work on the next day. That way I can hit the ground running.