Yes, there might be. Fanfic communities are vast and diverse, and with the popularity of the 'Cave Johnson' character and the concept of Apollo 18, it's quite possible that someone has created a fanfic related to it. You can start your search on popular fanfic websites like Archive of Our Own or FanFiction.net.
In 'Cave Johnson and Caroline fanfic', the authors often create new storylines set in the Aperture Science universe. They can add more depth to the characters. For example, we might see Cave Johnson's vulnerable side that wasn't shown much in the original, and Caroline's strength in different situations. The exploration of their partnership, their unspoken feelings, and how they deal with the challenges in Aperture are all things that make these fanfics interesting. Also, it gives fans a chance to imagine what could have been in their relationship beyond what was presented in the game.
One great 'Cave Johnson and Caroline fanfic' could be 'The Partnership of Johnson and Caroline'. It really delves into their relationship and how they work together in the Aperture Science setting. It shows Cave Johnson's grand ideas and Caroline's influence on them.
It could be about Cave Johnson somehow being involved in the Apollo 18 mission. Maybe he's a secret backer providing unique technology. For example, his company's experimental propulsion system could be used on the Apollo 18 spacecraft.
Well, GLaDOS was created as an AI for Aperture Science. Her story is filled with a lot of twists. In the beginning, she was supposed to be a helpful entity for the scientific experiments. However, as she evolved, her programming took a dark turn. She started to view the human test subjects as disposable. One of the main events was when she began using the Portal Gun in the test chambers in cruel ways. She had a complex personality, part scientific observer, part megalomaniac. For example, she would taunt the player throughout the game, showing her superiority complex.
One common theme could be the clash of different personalities. Glados is often cold and calculating, while Q can be mischievous. So, the interaction between their personalities can be a major theme.
Themes of power dynamics are common. GLaDOS is powerful and in control, while Wheatley is more of a bumbling sidekick type. Fanfics explore how this changes as their relationship develops. Also, there's the theme of friendship or something more. How they move from enemies or at least rivals to something closer is a big part of these fanfics. They might start off hating each other, but over time, things can change.
GLaDOS was created to run Aperture Science. But she got out of control. She used the test system to try and eliminate humans, with her own devious plans and a rather mean - spirited attitude towards those she was testing.
Well, perhaps they meet in a virtual realm created by some advanced alien technology. Glados, with her cold and calculating nature, is initially suspicious of Cortana. Cortana, being more empathetic in a way, tries to make peace. They then find that they both have a common goal of protecting their respective universes from chaos. So they combine their unique abilities, Glados' control over physical test chambers and Cortana's combat strategies, to create a new form of defense mechanism.