One idea could be Goku traveling back in time to train with his past self. He could teach his younger self how to achieve the SSJ God form more quickly, which would change the power dynamics in his original timeline battles. For example, against Frieza, if he had the SSJ God power earlier, the fight would be much shorter and less arduous.
A 'naruto ssj3 fanfic' is likely a fan - made fictional story that combines elements of Naruto and the Super Saiyan 3 (SSJ3) concept from Dragon Ball. It could be about a Naruto character achieving a form similar to SSJ3, perhaps with new powers and abilities, and the adventures or battles that follow.
You can try fan - fiction websites like FanFiction.net or Archive of Our Own. They usually have a wide variety of fan - created works, and there's a good chance you'll find 'naruto ssj3 fanfic' there.
Goku being born with SSJ4 in fanfic would likely make him a force to be reckoned with from infancy. This could result in quicker power-ups, different battles, and maybe even a shift in the balance of power among the characters.
Well, it's really hard to say. The Black Swordsman has his own set of skills with the sword and his combat experience. But Dark God Vegeta is extremely powerful with his Saiyan abilities. If they were to fight, it would depend on the context. If it's in a close - quarters combat, the Black Swordsman's sword skills might give him an edge initially. However, Vegeta could quickly adapt and use his energy blasts and super - strength to overpower. In a more open area, Vegeta's long - range attacks would be a big threat to the Black Swordsman.
One idea could be a crossover with the Naruto universe. Gohan as SSJ2 could end up in the Hidden Leaf Village and interact with the ninjas. Maybe he helps in a battle against a powerful enemy using his Saiyan powers.
One interesting plot point could be the initial encounter between the two. How would they react to each other? The Black Swordsman might be cautious seeing such a powerful being as Dark God Vegeta. Another point could be the discovery of each other's weaknesses. Maybe the Black Swordsman finds a way to temporarily nullify Vegeta's energy attacks, or Vegeta discovers that the swordsman's sword has a special power that can harm him. Also, the moment when they start to respect each other's fighting abilities during the battle would be really engaging.
One interesting story could be about a new villain who forces Goku to reach the SSJ4 form in a completely different way. Maybe he has to combine his Saiyan powers with some ancient energy source on a new planet. Another could be a fanfiction where Vegeta finally masters the SSJ4 form before Goku and has to face a powerful foe alone.