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.
One possible plot for this fanfiction could be that Bardock's return is not a simple one. He might be brought back by some sort of time - travel or resurrection magic. His return could stir up old rivalries among the Saiyans. And as he tries to fit into the current Dragon Ball Z universe, he discovers a conspiracy that threatens not only the Saiyans but the entire universe. So, he has to fight against this conspiracy while also trying to reconnect with his son Goku in a more meaningful way.
The plot could start with Cell having a change of heart after a near - death experience. Maybe he realizes the futility of destruction. Then he could start to learn about the values of the Z - fighters and try to fit in with them, while also using his powers for positive things like helping to train the younger fighters.
One possible interesting plot could be that Super 17 gains new powers that are not in the original series. Maybe he discovers a hidden energy within himself that allows him to time - travel. This could lead to him trying to change key events in the Dragon Ball Z universe, creating all kinds of chaos and new storylines for the other characters to deal with.
The plot could be about Bulma getting kidnapped by a new villain. Vegeta goes on a rampage to find her. He battles through hordes of minions until he finally reaches the villain's lair. There, he has an epic battle and rescues Bulma. Afterward, he realizes how much he truly loves her and vows to protect her better.
Maybe Naruto, in his godlike form, ends up in the Dragon Ball Super universe and teams up with Goku to fight a common enemy. They combine their powers in unique ways to defeat the bad guy.
One exciting plot could be that Goku and Naruto are brought together by some strange interdimensional rift. They initially misunderstand each other and get into a small fight. But then they realize they have a common enemy. An evil force is trying to collect the power of the Chakra and Ki to become all - powerful. So, they team up, with Naruto teaching Goku about chakra control and Goku showing Naruto some of his Super Saiyan forms and training methods. They then combine their powers in an epic battle against the evil force.
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.
One idea could be a story where Goku and Vegeta from Dragon Ball Super end up in the world of Dragon Ball GT by some sort of time - space rift. They then have to team up with the GT versions of themselves to fight a new, powerful enemy that is threatening both timelines.
A simple plot could be that Cabba and Lemon meet by chance. They then find out that they both have a common enemy. They start to train together to become strong enough to face this enemy. As they train, they share their knowledge and skills with each other. Eventually, they defeat the enemy and become heroes in their world.