You could leave the main character in a dangerous or uncertain situation. For example, have them about to face a big threat but not show how it plays out.
You could have a sudden event that changes everything and then stop. Say, the protagonist discovers a long-lost relative, but you don't reveal what happens next.
Introduce a new mystery or twist right at the end. Don't solve it, just hint at what's coming. This leaves readers eager for more. Like, have a character receive a mysterious letter and then the story ends.