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.
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.
Bulma could be an interesting main character as well. She's intelligent and resourceful. In a fanfiction, she could be the one who discovers the scientific properties of the 'lemons' in the crossover. She might use her knowledge to create new inventions or to help the Dragon Ball Super team in their adventure related to the lemons.
Super 17 himself could be the main antagonist if the fanfiction focuses on his corruption. He could be consumed by power and decide to rule over the universe. His already - formidable powers would make him a terrifying opponent for the Z - fighters, and they would have to find new ways to stop him without relying too much on their usual techniques.
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.
Goku is almost surely going to be one of the main characters. He's the face of Dragon Ball after all. Vegeta is also very likely, as his rivalry and friendship with Goku is a central part of the series. And of course, Bulma might be there, she's been important since the very beginning, providing the gang with all kinds of cool gadgets.
A new, self - created android by Dr. Gero's successor could be the main villain. This android could be specifically designed to target and defeat Super Saiyan Blue users. It could have the ability to absorb their energy and use it against them, making it a very difficult enemy for Goku and Vegeta to face.
It could be another powerful being from outside the known Dragon Ball universe. Maybe a being from a parallel dimension who sees the Grand Priest as a threat to his own plans for universal domination. Another option could be a rogue Angel who has turned against the Grand Priest and the current order of things in the multiverse.
It could be a new character created specifically for the fanfiction. Maybe a powerful alien warlord who has been observing the Saiyans and Broly from afar. He decides to test his strength against them by luring them into a trap. This antagonist could have unique powers, like the ability to manipulate time on a small scale during battles.
Goku could be the main character. Since he's the protagonist in the main Dragon Ball series, it would be interesting to see him explore the concept of Souls in the FighterZ universe. He could learn new ways to use his Super Saiyan powers in relation to his Soul.
Goku could be an interesting main character in a vampire fanfiction. Imagine Goku having to deal with the newfound thirst for blood while still trying to protect the Earth. His strong moral compass would be put to the test as he battles against his vampire instincts.