Take a look around
To come up with a hobby project or learning idea, you just need to look around. Something is always needed. For example, make it easier for yourself to do some everyday tasks with a program you write. It could be the future leader in downloads on the AppStore and Google Play.
Maybe it will create a new cloud drive to store your data or develop a new design for a famous website. According to the managers of BizUPLab company, if you like computer games, create a new one with your imagination and taste. Who knows, it might be a bestseller. Or maybe you're aiming to create a new programming language. Keep it simple at first, but you can improve it later. Also, a new social network, news aggregator, messenger, cataloguer, habit tracker, utility bot, payment integration, data visualisation, generative graphics, etc.
Many project ideas can be generated that will help you to work independently in your professional development. When you choose one, remember the main rule: the project you do should give you the opportunity to learn something new and make you happy with the end result.