Many (if not most) successful side projects are a success because they solve a problem that someone has. The first someone is typically "you". So if you're not solving a problem for yourself you won't spend any time on your projects. For example, A friend of mine scores little league baseball games every night and was tired of using a pencil and papper. He was interested in learning how to write an Android app, so he wrote one to keep baseball game scores. He has no intentions of releasing it, but assuming he wanted to, a big portion of the work is done already.

I think this is the reason why you see so many task and schedule planners. No-one has "solved" the problem of keep track of your tasks and checking them off. We have a bunch of people annoyed with themselves for forgetting things and writing new apps to solve the problem, from their own perspective.

So look for something in your life that you or a family member repeatedly does, which is boring, not productive and not fun. Write an app to make it productive and fun. Then if it works for you, it might work for others. They may also find it useful.