One idea could be a story about a young adventurer who discovers a hidden, magical world in the attic of an old, abandoned house. They meet strange creatures and have to find a way to get back home while also protecting the new world from an evil force.
How about a story based on real-life events that have had a significant impact on people's lives? It could be inspiring or thought-provoking. Also, a romance story with complex characters and their journey of love and growth is always popular.
A family mystery is also a great option. For example, the main character discovers an old letter in the attic that leads them to uncover long - hidden secrets about their family's past, like an unknown relative who was involved in a historical event.
Write about a time - traveling experience. A scientist accidentally activates a time - travel device and is sent back to a crucial moment in history. They have to figure out how to get back to their own time while not changing the course of history too much, which could lead to all sorts of interesting encounters and dilemmas.
Love is always a great topic. You could write about two people from different worlds falling in love. Maybe one is a mermaid and the other is a human sailor. Their love defies the rules of their respective worlds, and they have to fight against prejudice and the forces that try to keep them apart.