One possible fanfic could be about Goku's intense training before the Cell Games. He might be shown pushing his limits in the Hyperbolic Time Chamber, trying out new techniques and forms to defeat Cell. Maybe he discovers a new power level that no one, including himself, expected.
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.
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.
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 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.
The concept of 'cell drain tail' in dbz fanfic might bring together some interesting elements. The tail, being a unique part of Saiyan biology, could be the target or source of the cell drain. It could be that the cell drain is a result of some new form of bio - engineering or alien technology. There might be a focus on the physical and emotional changes the characters go through. For example, if a Saiyan's tail is being drained at the cellular level, it could cause them pain, weakness, or even strange mutations. And this could lead to new story arcs where the character has to find a way to reverse the process or use it to their advantage in some way.
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.
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.