Some facts about Apollo 13 are that it really faced a major crisis in space. There was an oxygen tank explosion which endangered the lives of the astronauts. Fiction might be any made - up stories about secret missions during that flight that have no basis in the real events that were documented.
The movie Apollo 18 is fictional. It doesn't draw from real-life events. It's a creation of the filmmakers' imagination to give viewers a thrilling sci-fi experience.
You might create a character who is a bit of an outcast among the gods. Maybe a minor deity who is neither fully on Apollo's side nor Cupid's. This character could be envious of the powers that Apollo and Cupid have. They could try to disrupt the affairs of the two gods in the fan fiction, adding an element of chaos and intrigue. For example, they might steal Cupid's arrows or try to deceive Apollo, which would lead to a series of interesting events.
One interesting storyline could be Apollo, the god of music and poetry, falling in love with someone Cupid has accidentally shot with his arrow. Cupid, realizing his mistake, tries to fix it but only makes things more complicated as Apollo's love is so strong it defies the normal rules of the gods.
Some fanfictions might also take a more creative approach. They could place Apollo in alternate universes, such as a world where the legal system is completely different. In this setting, Apollo Clay has to adapt to new rules and challenges, which makes for an exciting read as we see how his core values as a lawyer are tested in these strange new scenarios.
Apollo 18 isn't real. It was created for entertainment purposes and doesn't represent actual events. The story was made up to thrill and engage the audience.