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.
One scenario could be that Goku surprises Beerus with his speed and initial attacks as SSJ4. But then Beerus counters with his hakai and forces Goku to dodge constantly. Another could be that Beerus underestimates Goku at first and Goku manages to land some powerful blows.
Well, in many fanfictions, SSJ4 Goku is often depicted as having a chance against Beerus. SSJ4 Goku has shown incredible strength and combat skills. But Beerus has the power of a God of Destruction. His hakai ability could potentially wipe Goku out. Still, if the fanfiction is written in a way that gives Goku some new power - up or a special advantage, he could win. It really depends on the creative direction of the fanfic writer.
One possible storyline could be that Goku is accidentally transported to Alucard's world. They start off as enemies but then gradually learn about each other's powers and come to respect one another. Another could be that a powerful villain manipulates them into fighting each other. In this case, they might realize they are being used and team up against the real enemy.
It's really hard to say. Goku is extremely powerful with his Super Saiyan forms and incredible combat skills. But Alucard has his vampiric powers like immortality and the ability to control shadows. In a fanfic, it could go either way depending on how the author wants to write it. Maybe if the author wants to emphasize speed, Goku might have an edge. But if it's about dark, mysterious powers, Alucard could come out on top.
There could be a storyline where Issei challenges Goku to a friendly sparring match to test his own limits. As they fight, they exchange techniques and learn from each other. This could lead to Issei unlocking new forms of his own power, inspired by Goku's Super Saiyan forms. And Goku could also gain new insights into combat from Issei's more unorthodox fighting style.
It depends on the rules set in the fan - fiction. If it's a battle based on strategy, Issei might have a chance. He has shown to be quite cunning in his battles. But if it's a pure power - based fight, Goku has the upper hand. His years of training under various masters and his Saiyan heritage give him an edge. Also, Goku has experience fighting opponents much stronger than himself and finding ways to overcome them.
One common theme is the clash of different power systems. Star has her magic from the 'Star vs the Forces of Evil' universe, and Goku Black has his Saiyan powers from Dragon Ball. Another theme could be the exploration of good and evil. Goku Black is a villain, so stories might focus on how Star and her friends deal with his malevolence. Also, the idea of interdimensional travel can be a theme, as Goku Black is from a different universe in Dragon Ball and would have to cross into Star's world.
Well, I don't have a specific title off - hand. But a good place to start looking for 'Star vs the Forces of Evil Goku Black' fanfiction is Archive of Our Own. They have a wide range of fan - created works. You can filter by the relevant fandoms and look for stories that incorporate both elements in an interesting way, like maybe a story where Goku Black ends up in the dimension of 'Star vs the Forces of Evil' and causes chaos.