If you want to create something of high quality you should pursue something that you are passionate about or already have a lot of knowledge of. For example, I

If you want to create something of high quality you should pursue something that you are passionate about or already have a lot of knowledge of. For example, I like table tennis a lot, if I could work a few years on whatever I want, I would create some tool to improve the ways you can train in table tennis, for example create some TT robot that you can play against, or some AI to watch you play and show what you are doing wrong or how much spin/speed was on the ball. If you want to make something that's useful, don't make what everyone else is making or, if you do, make it A LOT better.