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.
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.
One possible plot twist could be that Naruto's closest friend turns out to be secretly working for the enemy. Just when Naruto is about to make a final stand against the invaders, his friend betrays him, leaving him in a difficult situation. But Naruto, being Naruto, manages to find a way to turn the situation around and still save the village.
In the 'Bardock Saves Raditz' fanfic, perhaps Bardock discovers a way to time - travel. He goes back to the moment when Raditz is about to die. Bardock could use his own power to shield Raditz from Goku's Kamehameha. Then, they team up and start exploring other planets, trying to find a new place for the Saiyans to thrive, away from Frieza's tyranny.
A plot twist might be that the oc has to team up with his enemies in the chaos realm to save Olympus. Or perhaps, when he saves Olympus, he accidentally unleashes another greater danger because of his chaotic powers. Maybe the gods who he saves end up betraying him because they fear his power as the son of chaos.
Well, a plot twist might be that when Naruto arrives to save Hinata, he finds out that Hinata has already made a deal with the Kumo ninjas for the sake of Konoha. So Naruto has to figure out how to honor Hinata's decision while still protecting her. Also, it could be that the Kumo situation is just a cover - up for a much larger conspiracy involving other villages. And there's a chance that the person Naruto thought was Hinata in danger was actually a clone, and the real Hinata is in an even more dangerous situation elsewhere.
One possible plot twist could be that the real danger was not what it seemed at first. For example, while the Avengers thought they were being attacked by a supervillain, it was actually a test set up by a greater power. And Peter discovers this truth while trying to save them. Another twist could be that Peter has to sacrifice one of his powers temporarily to save the Avengers, which changes his approach to being a superhero.
Perhaps the twist is that Sirius saves Draco, but in doing so, he sacrifices his own freedom. And then Draco has to figure out a way to return the favor and free Sirius. Or it could be that the reason Sirius saves Draco is not what it seems at first. Instead of pure altruism, he has a hidden motive related to his own past or a prophecy that involves Draco.
Another plot twist could be that the attacker was actually under the control of a mind - controlling villain. When Bucky subdues the attacker, the villain realizes that Bucky is onto him. So, the villain then tries to frame Bucky for the attack on the girl, forcing Bucky to clear his name while still protecting the girl from the real threat that the villain poses.
One possible plot twist could be that the troll is actually under the control of a dark wizard. So when Harry saves Hermione, they realize that there is a much bigger threat at hand. Another twist could be that the troll turns out to be a transformed Hogwarts student, and they have to find a way to reverse the spell instead of just fighting it.
In this type of fanfiction, a plot twist could be that Snape saves them, but they end up having to go into hiding in a completely unexpected place. Maybe they have to live among muggles in a way they never thought possible. Another plot twist could be that Snape saves them, but then realizes that he has put himself in a very dangerous position with Voldemort's followers. So he has to find a way to protect himself while also protecting Lily and James. And there could also be a plot twist where Snape saves them, but they don't believe his motives are pure and they try to escape from him, leading to a whole new set of adventures and challenges.