I haven't read this particular fanfic directly, but generally, in a 'bardock saves broly' fanfic, it might involve Bardock using his combat skills and intuition. Bardock, being a Saiyan, has a certain level of power and tenacity. He could potentially save Broly from some sort of danger, like a powerful enemy or a life - threatening situation on their home planet. It could also explore the relationship between the two Saiyans, perhaps Bardock seeing potential in Broly or having a sense of kinship towards him.
A possible plot twist could be that Bardock actually saves Broly, but in doing so, he unknowingly sets in motion a chain of events that leads to an even greater threat. For example, by saving Broly, he might disrupt the balance of power among the Saiyans in a way that angers some powerful factions, who then form an alliance with an external enemy.
A 'bardock vs king vegeta fanfic' could explore the backstories of both characters. Bardock's story as a Saiyan who starts to question the Frieza regime might contrast with King Vegeta's more traditional and perhaps self - serving rule. Their fight could be a symbol of the changing tides within the Saiyan community. It could also introduce new elements to the Dragon Ball lore, like different forms of Saiyan power that are unique to this fictional encounter.
One possible plotline could be that Broly, in his rage, accidentally provokes Beerus. Broly, not knowing who Beerus is, attacks him. Beerus, initially amused, decides to teach Broly a lesson but soon realizes that Broly's power is growing at an alarming rate. This leads to an intense battle where Broly pushes Beerus to use his full power. Another plotline could be that Beerus hears about Broly's power and decides to test him. He travels to Broly's location and challenges him. Broly, seeing Beerus as a threat to his friends or his planet, fights back with all his might.
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 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.
Another theme could be loyalty. Bardock might be loyal to the true potential of the Saiyan race while King Vegeta is loyal to the existing power structure. This difference in loyalty can create a great conflict in the fanfic. Also, there could be a theme of social change. As Bardock may represent the underclass Saiyans who want a change, and King Vegeta is the symbol of the old regime.
Well, they might be really shocked at the power levels of Goku and Broly. RWBY characters are used to their own kind of battles, and seeing the insane energy blasts and transformations in the Goku vs Broly fanfic could leave them in awe. They might start thinking about how they could incorporate some of those powerful moves into their own fighting styles.