A possible plot is that the OC is a magical being with the ability to transform Cell into different forms at will. This causes chaos in the DBZ world as Cell's new forms have unexpected powers. Another idea is that the OC is a spirit that can possess Cell and uses his body to achieve their own goals, which are not necessarily evil. For example, they might want to restore balance in the universe in a different way than the Z - Fighters. The last one could be that the OC is an alien from a planet that has a natural ability to nullify Cell's attacks. This makes Cell very frustrated and he becomes obsessed with either defeating the OC or learning their secret.
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.
Maybe the OC is a time - traveler who has come to prevent Cell from reaching his perfect form. They try to disrupt his plans at every turn. This could involve the OC teaming up with the Z - Fighters in some ways, and there would be a lot of action and strategy involved in their attempts to outwit Cell.
Maybe in the fanfiction, Cell could be re - created in a different form with a mix of other characters' DNA. He could have some of Piccolo's regeneration abilities along with Gohan's hidden power. This would lead to a whole new set of battles and power - level dynamics as the Z - fighters try to figure out how to deal with this new Cell.
Well, perhaps the OC is a scientist who was involved in the creation of Android 21 in some alternate universe. They could have a shared history that neither of them fully remembers at first. Or the OC could be a martial artist who challenges Android 21 to a fight not realizing her true power. This could lead to a series of events where they end up training together and learning from each other.
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 creating an OC who is a time - traveling gamer. They get sucked into the DBZ universe while playing the game. Another is an OC gamer who has the ability to level up like in a game within the DBZ world and uses that to gain unique powers.
One idea could be that Towa uses her time - related powers to create an alternate timeline where she and Goku have to team up against a new, powerful enemy that threatens both their worlds. This would force them to work together and understand each other's powers better.
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.
One idea could be that Frieza and Cell team up to take over the universe in a new and unexpected way. They combine their powers and intelligence to outwit all the other powerful beings.
One idea could be that Cell somehow gets a second chance at life and instead of being a villain, he tries to team up with Goku. They could go on adventures together across different universes, facing new threats and learning from each other. Goku could teach Cell about the value of protecting the innocent, and Cell could show Goku some of his scientific knowledge about power levels and energy manipulation.
One idea could be that Fem Buu ends up in the Naruto world and her unique abilities cause chaos. Naruto, being the hero, tries to understand her and figure out a way to send her back. Another plot could involve a team - up where Fem Buu and Naruto fight a common enemy that threatens both their worlds. Maybe there's a villain who wants to use Fem Buu's power for evil in the Naruto universe.