My suggestion is to make something you would find useful but that don't already abundantly exist (don't make a todo app or a calendar). This way you won't feel

My suggestion is to make something you would find useful but that don't already abundantly exist (don't make a todo app or a calendar). This way you won't feel frustrated if nobody uses it but you. Keep it simple, keep only the core and polish it. For ideas, I try to look into where I spend time during a typical day/week/month and see if I can find a way to simplify it or automate part of it. Then see if that's an issue other people have too by discussing with your friends, colleagues or making online searches on the topic. Don't get hung up on your idea if it's not useful to you. Try another one and write down your ideas when they come to you so you can revisit them later. The best ideas are usually the ones that keep coming back.