It could be a genetic mutation in the fanfic's storyline. Some strange event or factor could have caused a change in Bardock's genes, making it possible for him to become Super Saiyan. This genetic change might be related to some ancient Saiyan lore or a new element introduced in the fanfic's universe.
Well, in this fanfiction concept, Naruto going Super Saiyan 4 could also lead to some interesting character dynamics. His friends might be initially shocked and a bit scared of this new form. Naruto would then have to prove that he's still the same person despite this radical transformation. And as for the enemies, they would surely be caught off - guard and have to quickly come up with new strategies to deal with this super - powered Naruto.
One reason this type of fanfiction is so appealing is the potential for epic battles. Percy, with his sword and water - based powers, combined with the Super Saiyan God's energy attacks, could face off against some truly formidable foes. And not just in terms of physical combat, but also in strategic battles where he has to use both sets of powers cleverly. Also, the cultural clash between the two worlds can add a lot of humor and interesting plot points.
The change of Bardock becoming Super Saiyan in the Vegeta fanfic can completely rewrite Saiyan history. It could start a new era of Saiyan pride and power. Bardock, with his new - found Super Saiyan form, might be able to teach the younger Saiyans about the transformation. This could lead to a more united Saiyan race, as they all work towards achieving the Super Saiyan state. It might also mean that they are not as easily defeated by Frieza, as they have a stronger force to oppose him. This could result in a different galactic power structure, with the Saiyans being a more dominant force.
In the fanfiction, perhaps there is a prophecy that Naruto will become a Super Saiyan God - like being. As he gradually transforms, his body and mind change. He starts to see the world in a different way. His new powers attract the attention of both friends and foes. His friends try to help him adjust, while his foes try to either steal his power or destroy him. The plot could revolve around Naruto's journey to master his new form, protect his loved ones, and save the ninja world from destruction that only he, with his new powers, can prevent.
In this fanfic, the author could explore the origin of Broly's transformation into a Super Saiyan God. It might be a unique journey, different from how Goku and Vegeta achieved it. Perhaps there's a special training regimen or a hidden power within Broly that was awakened. The story could also delve into how the other characters in the Dragon Ball universe react to Broly's new form. Do they fear him? Admire him? And how does this change the power dynamics in the overall story? There could be epic battles against new foes that only Broly in his Super Saiyan God form can handle.
There's a fanfic where Bardock becomes a Super Saiyan God in an alternate universe. Here, he uses this power to try and change the fate of the Saiyans. The story could show how his newfound power affects his relationships with other characters like Vegeta and Goku, and how he might use it to stand up against the likes of Frieza in a different way than in the original story.