To quote Paul Graham, "Live in the future, then build what's missing."[0] In other words, think of something that seems like it should exist in the future because it will be useful (but doesn't yet, or does only in limited form), and then work toward that. If you're building something novel that solves a problem, other people trying to solve that problem will find you.
For example, I fantasized about being able to program and work in VR or AR in order to have an unlimited number of application windows around me in 3D space. So, I started by building an app for Android and iOS called VR Browser (https://apps.vuplex.com). Then, I started receiving emails from companies asking if they could license my technology to embed web content in their own VR apps and video games. Now I run a profitable business selling that 3D web browser functionality as a family of plugins for the Unity game engine (https://vuplex.com). Now that I have found product market fit, I just listen to my customers and they basically tell me (in aggregate) what to build next.
For example, I fantasized about being able to program and work in VR or AR in order to have an unlimited number of application windows around me in 3D space. So, I started by building an app for Android and iOS called VR Browser (https://apps.vuplex.com). Then, I started receiving emails from companies asking if they could license my technology to embed web content in their own VR apps and video games. Now I run a profitable business selling that 3D web browser functionality as a family of plugins for the Unity game engine (https://vuplex.com). Now that I have found product market fit, I just listen to my customers and they basically tell me (in aggregate) what to build next.
[0]: http://www.paulgraham.com/startupideas.html