It depends on what the 'first line story' actually is. Without knowing the specific first line, it's hard to say. But generally, it could be developed into a love story, an adventure story or a mystery story.
Since we don't have the exact 'first line story', let me give you an example. If the first line was 'In a small town on the edge of the forest, there was a strange old house.', the full story could be about a group of curious kids who decide to explore the house and discover some magical secrets hidden inside. Maybe there are some enchanted objects or a friendly ghost. They might face some challenges like traps or spooky noises, but in the end, they learn a valuable lesson about friendship and courage.
Well, if the 'first line story' is really short, say just a simple statement like 'The cat sat on the mat.', A full story could be that the cat is not an ordinary cat. It's a magical cat that can teleport to different places. One day, while sitting on the mat, it suddenly feels a strange pull and is teleported to a land full of talking mice. The cat then has to figure out how to get back home while dealing with these new and strange creatures. It could be a humorous story about the cat's misadventures in this new world.