Typically I just have an idea I want to try out, often from a software engineering angle, e.g. I once read the wikipedia article on maze generation algorithms, and I saw one I thought I could implement fairly easily in Unity, programmatically instantiating planes for the walls/ceiling/floor of each maze segment (the trick is to use the same material instance for each plane to get efficient rendering), and then I added a first person controller to navigate it, and then I thought it would be cool keep it dark and add suspense effets and a flashlight (I was playing Resident Evil at the time).
That was basically a one week project (with a lot of updates later) but as another example that ended up taking a lot more work, when the App Store opened up (way back around 2009) and Unity announced support for iOS, I thought the aspect ratio of iPhones was a match for the HyperBowl attraction game I worked on years earlier (2001) which displayed on a tall projection screen, and swiping on the screen could be a good substitute for the original bowling ball/trackball control, so I made a no-frills bowling game to test that control concept (a weekend project) and asked for a license to port HyperBowl, and then it took me a couple of years to get the original set of bowling lanes running.