Perhaps the OC is a bounty hunter who is after Riddick for a different reason than the other hunters in the movie. They start off as enemies but as they face the planet's deadly perils together, they develop an understanding and respect for each other. This could lead to a complex relationship where they both have to decide whether to continue their initial goals or form an alliance.
An OC could be a scientist sent to study the planet's ecosystem in the Pitch Black universe. They encounter Riddick and end up having to rely on his survival skills. The fanfiction could explore how the OC's scientific knowledge and Riddick's street - smart ways work together to overcome the various challenges on the planet, such as finding food and water sources.
One idea could be that the OC has a special power that Pitch Black is initially intrigued by and tries to use for his own dark purposes. For example, if the OC can manipulate dreams in a way different from the Guardians. Another plot could involve the OC being caught in the middle of the eternal struggle between Pitch and the Guardians, and having to make a choice about which side to ultimately support. Or perhaps the OC has a past connection to Pitch that is slowly revealed throughout the story, changing their relationship dynamic.
Well, if the OC is a mechanic, they could be on the planet to repair some valuable equipment. When things go wrong and they are left stranded, they use their mechanical knowledge to jury - rig weapons and defenses against the aliens. Their practical skills would be a great asset in the unforgiving world of Pitch Black.
One idea could be that the OC is a new student at Barden University and has an amazing, unique vocal talent that catches Donald's attention during auditions for the a cappella groups. Maybe the OC has a secret past related to music that gradually unfolds throughout the story, and Donald helps the OC overcome some insecurities related to it.
One idea could be to have the characters from Harry Potter end up in the Pitch Black universe. Maybe Harry and his friends could be stranded on a planet like in Pitch Black and have to deal with the dangerous creatures there while also using their magic in unique ways. For example, using Lumos to fend off the Pitch Black creatures in the dark.
One idea could be exploring Pitch Black's past in more detail. Maybe he had a friendship before he turned dark that could be rekindled in the fanfiction. Another idea is to have Pitch Black team up with the Guardians against a new, even greater threat.
Another plot could involve Rapunzel's hair having the ability to repel Pitch Black's nightmares. Pitch discovers this and tries to cut off her hair. However, Rapunzel uses her wits and the help of Pascal and Maximus to outsmart him. In the process, Rapunzel realizes she has more power within her than she ever knew, not just in her hair but in her spirit, and she becomes a force to be reckoned with against Pitch.
Another plot could revolve around Riddick trying to teach Jack some of his survival skills. He shows her how to track in the dark, use minimal resources for maximum effect. This could lead to a deeper bond between them as they face various challenges together. For example, they get trapped in a cave system and have to rely on each other to get out.
Well, a fanfiction could explore what would happen if Jack Frost was somehow influenced by Pitch Black's darkness. Maybe Pitch Black tries to corrupt Jack, but instead, Jack finds a way to redeem Pitch Black. Or perhaps in a different take, they meet in a dream realm where the normal rules don't apply, and they have a strange and intense encounter that changes their relationship.
A possible plot could involve Jack Frost accidentally stumbling into Pitch Black's lair. Pitch Black, instead of attacking him immediately, decides to play with him a bit. He creates illusions and challenges for Jack. Jack, being the adventurous spirit he is, takes on these challenges, and through this encounter, both characters learn something new about themselves and each other. In the end, it could lead to a change in their relationship, perhaps a more nuanced understanding between the two.
One idea could be exploring Pitch Black's past before he became the villain. Maybe he had a family or a lost love that led him down his dark path.