Luffy could change the past in several ways. Firstly, he could use his devil fruit powers in a more strategic way. Since he has the experience of future battles, he knows his opponents' weaknesses better. So, when facing the enemies who were involved in Ace's capture, he can defeat them more easily. Secondly, he could try to influence Ace's relationships. Maybe he can make Ace form stronger alliances with other pirate crews, which would make it more difficult for the Navy to target him. Also, Luffy could try to expose the Navy's secret plans earlier, which would change the power dynamics in the One Piece world and ultimately save Ace.
One possible plot twist could be that when Luffy goes back, he discovers that Ace's fate is more intertwined with the World Government's secrets than he thought. Maybe Ace was targeted not just because of his bloodline but because he knew something dangerous. Another twist could be that in the past, Luffy has to team up with unexpected allies, like some of the former enemies of the Straw Hats, to save Ace. And perhaps, his time - traveling disrupts the timeline in a way that creates new threats not only to Ace but to the entire world of One Piece.
One great fanfiction with this plot is 'Time's Redemption'. In it, Harry uses a powerful time - turning spell he discovers in an ancient book. He has to face many challenges in the past, like avoiding detection from the Ministry's time - turner regulations. Another good one is 'Back to Save Sirius'. Here, Harry teams up with Hermione and they use a secret time - travel device left by Dumbledore.
Well, perhaps Lilly realizes that Sev is in some sort of danger in the past that leads to a tragic end in the future. So she uses a magical device or spell to travel back in time. Once there, she has to find Sev among all the chaos and danger. Maybe she has to disguise herself to fit into the time period. Then she'd try to prevent the event that would have doomed Sev.
There might be a group of people who believe that Sev is dangerous because of his potential power. So they are trying to eliminate him. Lilly has to convince them otherwise or stop them from harming Sev.
One possible plot twist could be that Snape, in his attempt to save Harry, accidentally changes the entire future of the wizarding world. For example, he might prevent Voldemort from ever rising to power in the first place, which would mean that all the relationships and events that we know from the original story would be completely different. Another plot twist could be that Snape discovers that saving Harry is not as straightforward as he thought. Maybe there are dark forces at work that are trying to stop him from changing the past, and he has to fight against them while also trying to protect Harry.
She could meet her younger self. This would be an interesting encounter as she could offer advice and guidance to her younger version. Maybe she could help her younger self to be less afraid of her powers.
Character development is key. Fanfictions explore how Ace and Luffy grow under Garp's influence. Maybe Luffy becomes more disciplined in some stories, or Ace learns to be more open - hearted, all thanks to their interactions with Garp and each other.
One great 'Ace Sabo Luffy Fanfiction' is 'The Brothers' Bond'. It beautifully explores their relationship, showing how they support each other through various challenges. Another one is 'Sabo's Return'. It focuses on Sabo coming back into Luffy and Ace's lives and how they all readjust. There's also 'Ace and Luffy's Adventure', which takes the three on a fictional journey filled with action and heartwarming moments.
I'm not entirely sure what 'oc' specifically refers to in this context. If it's some kind of application or platform, you should first look for options like 'download' or'save as' within the interface related to the ace fanfiction. Maybe there are specific folders or storage areas where it can be saved.