If Goku was brought back after the Cell Games, it could have led to unexpected alliances or rivalries. Also, his presence might have prevented some future villains from emerging or made their plans much more difficult.
It could have led to a much quicker victory for Goku. His SSJ2 form is way more powerful than SSJ1.
It could be someone from Goku's past who has a vendetta against him and takes advantage of the chaos during the Cell Games. For instance, an old enemy who was thought to be defeated but managed to come back stronger and wants to finish Goku off while he's weakened from the fight with Cell.
It could have led to all sorts of interesting plot twists. Maybe they would have had powerful kids or faced unique challenges together.
One possible scenario could be that Goku discovers a new power within himself during the Cell Games that allows him to not only survive but also defeat Cell more easily. Maybe it's a hidden form that he unlocks at the last moment.
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.
One interesting concept could be new power - ups for Goku. Maybe he discovers a unique form during the Cell Games in the fanfic.
Well, Goku is surely one of the main characters as it's related to the Cell Games. He is the central figure in the fight against Cell.
The story could progress with a darker tone. There would be a lot of mourning at first. Then, the remaining characters might start to look for ways to prevent future threats more aggressively. For example, Bulma could use her scientific skills to develop new weapons or defenses.
It could have led to a completely different story arc. Maybe they would have teamed up and faced even greater challenges together.
Their relationship might start with distrust. Goku would be wary of a being related to Cell. But if the female Cell shows good intentions, Goku could gradually accept her. Eventually, they could become friends or even more.