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.
The main differences are in their expressions and some minor details of personality. In the manga, Deku might seem a bit more intense, while in the anime, he's sometimes presented a bit softer.
Well, Vegeta would be a likely character. He might be initially suspicious of the female Goku. Maybe he thinks she's a threat or just can't believe there's another Goku - like being. He could be there to add some comical moments as he tries to understand the situation.
Well, obviously Deku in two forms - the younger and the older version are the main characters. The younger Deku is the one who is still growing and learning, while the older Deku has more experience.
In general, it's a bit of a toss-up. The strength of Manga Goku and Anime Goku can vary depending on the specific arcs and battles. Sometimes, it might seem like one is stronger in a particular situation, but that could change in another.
Another plot might involve the older Deku being from an alternate timeline. He shows the younger Deku different ways he could have developed his skills and relationships, which makes the younger Deku reevaluate his own path as a hero.