One acceptable theme is their relationship in terms of power balance. How Piccolo's different forms compare to Cell's various stages of evolution. Also, the psychological aspect of their encounters, like how they perceive each other as opponents, can be a great theme for fanfiction.
Writing such fanfiction that involves inappropriate and non - canon sexual content is not appropriate. Dragon Ball series is mainly focused on action, adventure, and the development of characters and their powers, not this kind of vulgar content.
One possible plotline could be that Cell, seeking to prove his superiority, challenges Broly. Broly, in his usual berserk state, accepts without hesitation. The fight could start with Cell trying out his various attacks like the Kamehameha - like Solar Flare and his physical strikes. But Broly's power is just too much and he starts to overpower Cell. However, Cell then reveals a new, hidden ability that he has been developing in secret and the tables are turned for a while.
I'm not entirely sure which specific 'Cell 13' story you're referring to. There could be many stories with such a title. It might be a mystery about something that happened in cell 13, perhaps a prison cell or a cell in some sort of institution. Without more context, it's difficult to say more.
McGregor's unruly behavior and his disrespect towards Khabib. He said things that were completely unacceptable. Khabib, being a proud fighter with strong values, couldn't let those remarks slide. So, he took on McGregor in the fight to set things straight and prove his worth in the MMA world.
Well, it really depends on how the fanfic is written. Broly is incredibly powerful with his insane strength and rage - fueled power. But Cell is also no slouch, being a bio - engineered super - being with a wide range of abilities. If the fanfic focuses on brute strength, Broly might have the upper hand. But if it's about strategy and adaptability, Cell could come out on top.
There was a girl and a boy in a competition. It was a race. The boy was overconfident and made some taunting remarks towards the girl before the race. This made the girl determined to win. During the race, the boy tried to block the girl's path a few times. The girl, not one to be bullied, elbowed him gently but firmly out of the way. In the end, the girl won and the boy was left fuming. But the girl just walked away with a proud smile on her face.
Once, I saw a wife and a mistress have an argument in a park. The wife found out about the mistress and confronted her. The mistress tried to act tough at first, but the wife was full of anger and rightful indignation. The wife started shouting about how the mistress was ruining her family. In the end, the mistress, feeling ashamed, ran away.
It could be a story about a scientific experiment gone wrong in a cell labeled 13. Maybe there were some unforeseen consequences and the people involved are trying to solve the problems that arose from it.