We don't have to start from scratch when creating gestures. Humans around the world have a rich set of gestures that they use on all sorts of occasions. These "premade" gestures are easy to learn and use and can be enhanced with interactivity to create rich, meaningful interactions.
There are two basic types of gestures: static and dynamic. Static gestures are postures, meaning they are performed and held. The "okay" sign is a static gesture. Dynamic gestures are more about movement over time. Twirling a finger is one example. As a general rule, dynamic gestures are much more difficult to detect and pattern-match in code than static gestures.
Sign language is a means of communication that is typically used by the hearing-impaired and employs gestures and facial expressions instead of sound to fluidly express thoughts. Using sign language for interactive gestures is not recommended because not only are the signs complicated (as they have to convey a wide range of expressions), but also they vary based on the underlying vocal language. Such gestures could also interfere with actual sign-language speakers.