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.
A plotline could involve an outside force manipulating Broly and Cell into fighting each other. This outside force could be another villain who wants to see them both weakened so they can take over. Broly and Cell initially fight each other, but then realize they are being played. They could either continue to fight or team up to take on the real villain. Another idea is that a group of Saiyans or other Z - Fighters bet on the outcome of the Broly vs Cell fight, which adds an interesting side story to the main battle.
A plotline could involve Broly being sent to Kale's universe by some sort of interdimensional rift. Kale initially sees him as a threat and attacks. But as the fight progresses, they start to understand each other's power and motives. Or perhaps a fanfic could have a story where both Broly and Kale are training on the same planet. Their training methods clash, leading to a fight where they both learn from each other and grow stronger in the process.
A plotline could start with Frieza hearing about Broly's power and being jealous. He then devises a plan to ambush Broly. But Broly, being extremely perceptive in his own way, senses Frieza's approach and is ready for the fight. As the fight progresses, Frieza realizes that Broly is much stronger than he expected and has to use all his dirty tricks to try and win. Or perhaps the plotline could involve Broly being captured by Frieza's forces, and then Broly breaks free and goes on a rampage against Frieza and his minions.
One possible plotline is that Broly mistakes Kale for an enemy at first. But when he realizes she's also a Saiyan, he holds back. They then start to compare their power levels. Another could be that they are both sent on a mission together by some higher - up Saiyan authority. On the mission, they face various challenges and come to respect each other's abilities.
One possible plotline is that Broly is brought to the Harry Potter world as a result of a powerful spell gone wrong. He initially causes chaos with his strength but then gradually learns about the concept of magic and starts to respect it. Another could be that Broly is a hidden protector of Hogwarts, his Saiyan powers being the ultimate defense against a new dark force that the normal magic in the wizarding world can't handle.
A plotline could involve a special Pokemon that both Ash and Paul are interested in. They might compete to catch it, which could involve a series of challenges and battles. Or, perhaps there's a new type of Pokemon training technique that one discovers and the other tries to learn from or outdo. This could create an interesting dynamic in the fanfic.
Another plotline might be that the forces of evil launch a sudden attack on Comet's homeland. Comet has to fight back and in the process, uncovers a conspiracy within the forces of evil that could be used to their advantage. He then uses this knowledge to turn the tables on them.
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.
One possible plotline could be that the starless star is on a quest to find its true identity and in the process has to battle the forces of evil. Another could be that the forces of evil are trying to destroy something precious and the starless star is the only one who can stop them.
One possible plotline could be that the Lake Trio discovers a threat to the world that Arceus is either unaware of or is causing (unintentionally perhaps). So they have to battle Arceus to stop this threat. Another could be that a powerful villain manipulates the Lake Trio into believing that Arceus is a danger and they must fight it.
A possible plotline could be a competition between the two camps. Maybe there's a prophecy that says the winning camp will gain some great power or avoid a terrible disaster. So the demigods from both camps have to compete in various challenges, like races, combat, and tests of wits. Another plotline could be a misunderstanding that leads to conflict. For example, a theft at one camp is wrongly blamed on the other, and they start to build up hostilities.