conlanging
Conlanging may arguably not be a field of linguistics, but it is a great job opportunity. Furthermore, it's a job that not many people think of when they're watching movies and TV shows with non-Earth languages - sometimes the writers of the series use gibberish or even an obscure language that not many people will recognize, but sometimes, the writers hire a linguist to construct a language for them.
This is where the term "conlanging" comes from. Conlang stands for "constructed language". And although conlanging seems to be a relatively new term and a new field for linguists, it actually has a longer history than you think.
One of the first and most famous conlangers is none other than J. R. R. Tolkien, the author of The Silmarillion, The Hobbit, and The Lord of the Rings. His languages Quenya and Sindarin are among the most famous constructed languages and have one of the most documented grammars and vocabularies of any constructed language.
Other famous constructed languages include Klingon (Star Trek), Na'vi (Avatar), and Dothraki (Game of Thrones). However, constructed languages do not just include languages made up for fantasy settings, but also auxiliary languages. An auxiliary language is a language invented for international communication. The most famous auxlang is Esperanto.
Conlanging is hard work, as it takes a lot of research to properly construct a believable language, especially if it's a language not based on any other existing languages (an a priori language) as opposed to being based on an existing language (a posteriori).
In his book, The Art of Language Invention, author David Peterson describes how to construct a language. David Peterson himself is credited with fleshing out the languages of Dothraki and Valyrian from the Game of Thrones TV series (which are themselves based on the book series, A Song of Ice and Fire by George R. R. Martin) as well as the language of the Dark Elves in Thor: The Dark World. If you'd like to look into constructing your own language (or even getting a crash course in linguistics itself), I highly recommend reading his book!
Information from The Art of Language Invention by David J. Peterson.