Picture planets floating close together in space, within reach of your giant, disembodied hand. If you would rather keep the laws of physics intact, you are standing in a black room, and all around you, planet-spheres of various colours hang from the ceiling. These are the worlds of your imagination.
Some are just like our own, the only difference your fictional events. Others, you opened and tweaked, because the world can always be better, or worse in more interesting ways. Then, there are the brand-new orbs, or perhaps they are disks, or even cubes or Dyson spheres, and while they bear resemblance to those we know, you moulded them into something entirely yours.
If ever you wonder where to get ideas, they are all within reach. Maybe you dismissed them as daydreams, but daydreams are ideas in embryonic form. To flesh them out into worlds, into the beginning of a narrative context, let them simmer for a while. Think on them throughout the day, throughout the week. Throw characters into this primeval soup. See if a plot is forming. Then, hammer it out into a coherent concept for a story.
But if you never find yourself thinking of a fictional character of your own, if you never come up with a silly scenario and tell yourself it would be so cool or so funny, you may be in a funk. Read books, watch shows and movies, listen to history and science podcasts (my current recommendation is The British History Podcast). Take in narratives and world-building elements to oil the gears and get them turning again. Find moments to rest, because a fatigued brain does not do much conjuring. And do not be so quick to dismiss the fancies that may pop into your head.
All the worlds are yours to touch. As I write my first series, I already have plans for a standalone novel and a completely different series, plus a wouldn’t-it-be-cool concept or two floating around in there, waiting to be more clearly defined. Let it happen.