Mystery/Thriller:
* The "Smiling" Killer: The protagonist, a dental hygienist, discovers a pattern in the deaths of her patients - they all had perfect, unnervingly similar smiles. She suspects a serial killer, but no one believes her. The cliffhanger is the realization that the killer is someone close to her, someone she trusts.
* The Missing Teeth: A seemingly mundane dental appointment turns into a nightmare when the patient's teeth mysteriously disappear, leaving only a bloody gap in their mouth. The protagonist is left wondering: Was it an accident, a bizarre medical anomaly, or something far more sinister?
Romance/Drama:
* The False Smile: The protagonist, a shy and insecure person, is offered a job at a company that specializes in creating "perfect smiles" using a controversial new technology. They are hesitant but ultimately agree, undergoing the procedure themselves. The cliffhanger is the realization that their newfound confidence and attractiveness might come at a terrible, unforeseen price.
* The Unrequited Smile: The protagonist has been secretly crushing on their dentist for years, but they are too shy to admit it. They finally gather the courage to make a move, only to discover that the dentist is already involved with someone else. The cliffhanger is the heartbroken realization that they may never have their chance.
Science Fiction/Fantasy:
* The Smile's Secret: The protagonist discovers that a mysterious smile etched on an ancient artifact holds the key to unlocking a forgotten power. But, as they delve deeper into the secrets of the smile, they find themselves facing a terrifying choice: use the power for good or succumb to its corrupting influence.
* The Living Smile: A rogue experiment in facial reconstruction results in the creation of a sentient smile that can move and communicate on its own. The protagonist is tasked with stopping it before it can spread its influence, but they soon realize that the "smile" has its own agenda.
Remember to consider the tone and genre of your book when deciding on your cliffhanger. A good cliffhanger should leave the reader wanting more, but it should also feel natural and organic to the story.