I'm not entirely sure specifically what a 'beerus raps beerus fanfic' is about without more context. It could potentially be a fan - created story where the character Beerus is rapping. Maybe it involves Beerus expressing his thoughts, his powers, or his experiences in the form of rap within the context of a fan - made fictional narrative.
One idea could be a crossover with the Dragon Ball Super universe and the DC Comics universe. Zamasu could team up with Darkseid, believing they share a similar view of bringing 'order' to their respective realms. Another idea is a crossover with the Naruto universe, where Zamasu tries to impose his will on the ninja world, facing off against powerful shinobi like Naruto and Sasuke. Or perhaps a crossover with the Star Wars universe, with Zamasu as a Sith - like figure, trying to gain control over the Force.
You could try looking on fan - fiction websites like Archive of Our Own or FanFiction.net. They usually have a wide variety of fan - created works, and there's a good chance you might find a 'beerus raps beerus fanfic' there.
One interesting Zamasu fanfic could be about his origin story in more detail. It might explore his early days in the Kaioshin realm, his training, and how his ideology started to form.
Zamasu meets his end in the manga because he underestimates the strength and strategy of Goku and Vegeta. They exploit his weaknesses and deliver the final blow that finishes him off.
Beerus would likely be more powerful. He is a god of destruction in his own universe with incredible cosmic powers. Naruto is extremely powerful in his world, but his power is mostly based on chakra and ninjutsu, which is on a much smaller scale compared to Beerus' ability to destroy planets with a flick of his finger.
Well, Naruto could be in awe of Beerus' power. Maybe he tries to learn some of Beerus' combat techniques. Since Naruto is known for his determination, he might pester Beerus to train him.
With King Kai, Beerus as a girl could have a different kind of relationship. She might be more patient with him. King Kai might be a bit more nervous around her at first, but then they could develop a more comical and friendly rapport, with Beerus teasing him in a more light - hearted way.
The idea of an alternate universe is also very common. Since Zamasu is from a different franchise, fanfictions like to create a new world where both Naruto and Zamasu co - exist. This allows for new character relationships to be formed, new storylines to be developed, and for the writers to be really creative with the setting and the overall plot.