Obviously, Cell himself is one of the most popular characters. Since the fanfic is centered around him, fans love to explore his character further. Goku also remains popular as he is often pitted against Cell in these fanfics. Vegeta is another one. His rivalry with Goku and his own quest for power make him a great addition to any story involving Cell. And then there's Trunks, who has his own unique abilities and backstory that can be intertwined with Cell's story in interesting ways.
Well, one interesting concept could be new power - up forms for the characters. For example, a fanfic might introduce a unique form for Cell that combines the abilities of different Saiyans in a completely unexpected way. Another concept could be alternate timelines where Cell has different fates. Maybe in one timeline, he becomes a hero instead of a villain. And also, there could be concepts about Cell's family or origin that are explored more deeply, like his creator having some secret motives that are not shown in the original series.
In this fanfiction, perhaps Cell returns with an army of his own clones. He could be seeking revenge on the Z - fighters for his previous defeat. The story might focus on how the heroes train to face this new threat from Cell. It could also introduce new characters who team up with the Z - fighters to take on Cell and his clones. There could be a lot of intense fight scenes and power - up moments.
It might still be Goku. Since Cell is coming back, Goku would be the one to step up and defend the Earth once again. He would be at the center of the fight against Cell.
Many Dragon Ball Z Cell fanfics might explore alternate timelines. For example, what if Cell had successfully absorbed all the Androids before facing Goku? This would have made him even more powerful and could have led to a completely different outcome for the Dragon Ball Z universe. It's exciting to think about all these different possibilities.