General:
* Lengthen: This is the most direct opposite of shorten.
* Extend: Similar to lengthen, this implies making something longer or greater in duration.
* Prolong: This suggests making something last longer.
Specific:
* Expand: If you're shortening a text, the opposite might be to expand it.
* Enlarge: This could be the opposite of shortening a physical object.
* Increase: This could be the opposite of shortening a quantity or duration.
Other possibilities:
* Amplify: This could be the opposite of shortening a sound.
* Strengthen: This could be the opposite of shortening a rope or a piece of fabric.
The best antonym for "shorten" depends on the specific situation and the meaning you want to convey.