The phrase "drop a line" is often used to mean sending a brief message, letter or note to someone. However, there are several antonyms for this phrase such as "write a novel," "draft an essay," "craft a long letter," "compose a lengthy missive," or "pen a detailed message." These antonyms highlight the idea of a longer and more comprehensive communication, which goes beyond just a quick message. Such antonyms may be more suitable for formal communications or when conveying more complex ideas or emotions. Ultimately, the choice of antonym depends on the purpose and context of the message being conveyed.