Just a prompt is enough for me to build an entire story. *Our Universe was actually a short story I wrote for a writing prompt in a discord server:
When someone's heart breaks so does a piece of our world. This creates fissures, valleys and even cracks in the pavement. Tell a story about one of the broken pieces in this world.
I made it a short cosmic horror romance story, and then I was like "this could be a comic". So I did that. It was supposed to be a oneshot as well. But then it just got bigger and bigger. Now I'm working on ending season 2 and leading into season 3.
I will say that even vague concepts can help with a story. The vaguer or the stranger, the better. Hell, I've been working on a comic based on the fact that butterflies also drink blood. I ended up doing concept art of a woman with butteflies in her hair because she's dead. And then I thought -- "what if she was a ghost or back from the dead?" Boom -- another cosmic horror story.