Power is a very common theme. Fans often explore who would be more powerful in different situations or how they could combine their powers. Another theme is rivalry, as they are both powerful villains in the Dragon Ball universe. Their personalities also play a big role, with themes of pride and ego often coming up.
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.
It could be another powerful villain from the Dragon Ball universe like Cell. Since Gohan and Frieza are starting to form an unlikely bond, Cell might see this as a threat to his own plans of domination. He could try to break them apart and defeat them both.
One idea could be that Frieza, after being resurrected yet again, finds himself in a situation where he has to team up with Gohan. During their time together, they start to understand each other's motives and past traumas. Gohan's kindness and sense of justice gradually influence Frieza in unexpected ways.
I'm not sure about a specific 'cell x frieza fanfic' title off the top of my head. However, you can check out fanfic platforms like FanFiction.net or Archive of Our Own. Just search for 'cell x frieza' in their search bars and you'll likely find a bunch of stories. Some might focus on their power dynamics, like who is stronger in different scenarios, while others could explore a more friendship - based relationship between the two.
One great 'Goku x Reader Fanfiction' is 'A Saiyan's Heart'. It really delves into the relationship between Goku and the reader. The story shows how Goku's pure-hearted nature affects the relationship and how the reader grows to understand his world of fighting and protecting. Another interesting one is 'Goku's Unseen Bond'. It takes a unique approach by placing the reader in the middle of Goku's training sessions and adventures, creating a very engaging story.