A common theme is power. Since Cell is a powerful character in DBZ, often in 'cell x oc dbz fanfic', the OC might be on a journey to gain power or deal with Cell's power. Another theme is redemption. Sometimes the fanfic might explore Cell's character in a way that gives him a chance at redemption through his relationship with the OC.
One idea could be that the OC is a powerful being from another dimension and Cell is intrigued by their power. He tries to either absorb them or form an alliance to take over the universe together. Another plot could be the OC has a special ability to control Cell's cells, creating an interesting power dynamic between them. And a third could be that the OC is a time - traveler who knows Cell's future and tries to change his path in the DBZ universe.
First, give the OC a unique power. For example, the ability to manipulate energy in a way that no one in the DBZ universe has seen before. Second, give them a distinct backstory. Maybe they come from a long - lost race that was wiped out by Frieza's ancestors. And third, give them a unique personality. Instead of being the typical heroic type, make them morally grey. They might be willing to team up with Cell if it means achieving their own goals.
Well, one way to find good 'cell x oc dbz fanfic' is to check out popular fanfic websites like FanFiction.net or Archive of Our Own. Just search for the 'cell x oc dbz' tag and browse through the results. Some might have really interesting takes on the relationship between Cell and an original character in the Dragon Ball Z universe.
First, give the MC a clear backstory. For example, if they are a human, explain where they come from, their family, and what led them to get involved with the events related to Cell. Maybe they were training in martial arts because they had a personal vendetta against powerful beings after their family was killed by a random villain. This would give them a motivation to face Cell.
The theme of transformation is also popular. It could be a physical transformation of the female Cell, or a more emotional and mental one as she navigates the complex world of DBZ, often changing from a more villainous or neutral stance to a more heroic one.
One interesting fanfiction could be where Cell somehow ends up in the Naruto universe. Maybe he's a new type of enemy that the ninja have to face. It could bring a whole new level of danger as his powers are different from the normal jutsu - based attacks in Naruto.
One good dbz x oc fanfiction could be 'The New Saiyan'. In it, the OC has unique powers that interact in interesting ways with the DBZ characters. It's full of action and adventure.
If Goku and Female Cell interacted, it could change the perception of enemies in the DBZ universe. Their cooperation might lead other fighters to be more open to the idea of redeeming former foes. For example, if they manage to work together successfully, Vegeta might rethink his stance on some of the villains he's faced.
One idea could be to explore what if Gohan had a different training partner before the Cell Games. For example, if Piccolo had trained with him more intensively, it might have changed his power level and fighting style during the games.