When the Tides Come In (formerly titled Could Have Been) is a One Piece Fan Fic by Delta Wish. In an alternate universe, the Marines are losing a war. In an attempt to turn the tables, they find a way to bring the strongest man from another world to help them. What they did not expect was Monkey D. Luffy to come from the wormhole, especially when to them, he's just a skinny brat clinging to his Grandfather.
A story where Pirate King!Luffy meets Marine!Luffy.
- Berserk Button: Akainu's mere presence is enough to piss off PK!Luffy. And when PK!Luffy found out what happened to Vivi and Robin, Crocodile was screwed, no questions asked.
- Curb-Stomp Battle: Every battle PK!Luffy has been in. His battle with Crocodile is so one-sided that it's explicitly called a beating, and his battle with Blackbeard lasted all of five minutes.
- The Dreaded: PK!Luffy, in both worlds. His killing intent was enough to paralyze Fleet Admiral Sengoku, the Admirals, and leave several vice-admirals near tears. His haki was strong enough to knock out vice-admirals easily, and controlled enough to leave one person conscious so he could beat the shit out of him. Back in his own world, no one is willing to get on his bad side. PK!Luffy's Crocodile is so scared of him that he deliberately stays out of his way and avoids Alabasta like the plague, knowing that if he went anywhere near Vivi or her country, Luffy would kill him.
- For Want of a Nail/What Could Have Been: This story explores a reality where Luffy was raised in Marineford by his grandfather instead of in Foosha Village and Mt. Corvo with Ace, Sabo, and Dadan like in canon. Needless to say, the two worlds are vastly different.
- Killed Off for Real: Because M!Luffy did not take part in the Alabasta Civil War, Crocodile won and both Vivi and Robin are dead.
- No-Holds-Barred Beatdown: What PK!Luffy did to Crocodile in M!Luffy's world. He didn't even bother using his devil fruit powers or hitting as hard as he could have. It was an explicit, haki-empowered, bare-handed beating. The only reason Crocodile survived long enough to reach Impel Down is because they called a doctor.
- Noble Bigot with a Badge: M!Luffy is notably prejudiced against pirates, to the point where he is stunned that a former pirate cares about his family, and hates the fact that in another reality, he's a pirate. Nonetheless, this is due to the experiences he saw where pirates committed acts of the evilest kind (which the majority of pirates actually do). At the same time, he's obviously blind to the injustices that the Marines commit in the name of the World Nobles — no doubt this is due to being somewhat sheltered from his overprotective grandfather, making PK!Luffy far more world-weary than M!Luffy. Rika even calls him out on it. It's also implied that he is unaware of the identity of his father — which explains his near-idolization of his grandfather, the only parental figure he had in his life.
- Rank Up: PK!Luffy reveals that his Coby is now Fleet Admiral.
- Spared By Adaptation: Ace is alive in M!Luffy's world. Sabo counted up until the point that canon revealed that he was still alive; since this fic was published beforehand, it makes the assumption that Sabo did in fact die during Luffy and Ace's childhood. Although the author did make a quick edit after Sabo's survival was revealed, and in a way still counts in the fact that Sabo was not separated from Ace when they were young.
- World's Strongest Man: Pirate King!Luffy, aka canon Luffy, which is why the Marines asked for his help.