Every story starts somewhere, and sometimes the best stories run parallel or perpendicular to their
starting point. Think Run Lola Run. Or this from a BOMB interview with César Aira:
If a little bird enters into the café where I’m writing—it did happen once—it also enters into what I’m writing. Even if a priori it doesn’t relate to anything, a posteriori I make it relate.
For example, if I’m writing a scene about a couple, a marital spat in a house with closed windows and doors . . . . So, I make the bird appear flapping around among the furniture, and I find a way for the bird to have a reason for appearing in the story. It could be a mechanical bird designed by an engineer who was the woman’s first husband, whom her present husband thought was dead, but the engineer faked his own death to escape justice—he had invented killer mechanical pigeons. He continues to live under a false identity, and she’s discovered him and is blackmailing him . . . . It could be this or anything else. In spite of all my admiration for Surrealism and Dadaism I never liked the mere accumulation of incongruous things. For me, everything has to be sewn together in a very conventional fashion. I always think of something. And what I think of also changes the course of the plot. Since the next day something different will happen at the café, the plot continues to change accordingly. That sinuous thread in my novels is more interesting to me, more writeable, than a linear plot.
Think of a story, subject, or character. Write that story or idea. Now write about another moment; nothing significant, just a starting point. Keep writing until the stories meet.