(Carrier AU. So here's another idea because apparently the fun doesn't stop ;P Anyway so the plot for this one is this: There was a terrible chemical war during One Piece's history, I'd probably put this during the Void Century but the timeline doesn't really matter for it I don't think, unless my plot suddenly changes at any rate. So anyway the chemicals affected woman's wombs, tainting many and leaving them unable to become pregnant and it caused a serious crisis and a rapid decline in the population, so badly that sometimes it was only one or two children on and island at any one time. However one day some priestess on one of their sacred travels found these crystals that glowed around certain people. After a while they discovered it was people who could become pregnant, some men, woman and other different types of species. The faithful grew more powerful with each crystal they harvested from their sacred travels and the temples grew in influence. Now the priestess are a common sight, carriers are still rather rare but not as before thankfully, unfortunately though many are dragged off to the temples because they are a 'gift from God' and must be protected but there are also many who wish to avoid this.)

"Hey Makino!" Luffy called out during dinner where all of them sat around a table.

It was rather nice and calm, Makino visiting always had that effect. Everything would just settle down.

"Yes Luffy? Makino asked as she poured Luffy out some milk.

"Ace and I saw these weird people out with these crystals at the town square today! The crystals were so shiny!"

Makino nodded. "Ah yes, well it's to tell if someone is a 'carrier' or not."

"A carrier?" Luffy blinked.

"Do you not remember the talk I gave you Luffy?" Makino asked gently.

"Carriers are people who can carry a baby." Ace reminded him, still earning a blank stare. "People have sex and if they are a carrier then they carry a baby."

"They become pregnant." Makino corrected with a warm smile. "It used to just be woman who carried children once upon a time but from what the priestess' from the Temples say something happened that harmed the wombs that all women used to have. For a long time there were rarely any children around, it would be rare for a child to have a sibling like you do Luffy."

"That's horrible!" Luffy gasped, instantly latching on to Ace. "I'd hate that!"

"Apparently though the priestess' discovered that only certain people could carry children. They noticed that these strange crystals would light up whenever there seemed to be a carrier around and after thorough testing they discovered the crystals would only light up around them, men, women and all sorts of other different species out there. The faithful declared it a gift from God and the World Government agreed. Once someone is found out to be a carrier they are put in the faithful's records." Makino smiled as she began to fish through her bag. "I got a spare one in case you two wanted to see for yourselves."

"The shiny thing!" Luffy beamed, reaching for the shining stone in Makino's hand.

Dadan raised an eyebrow. "I'm not surprised you're a carrier. You suit it."

"Thank you! I'm excited that one day I might get to have a child." Makino beamed before glancing over the youngest sibling. "So was it shining around you Luffy?"

"We didn't get close to it but it sometimes shined around other people whenever I saw those faithful people holding them out." Luffy explained, frowning when the crystal didn't light up in his hands. "Aww..."

Ace frowned. "Shame it's not glowy. It sparkles when it does."

"Here Ace! You hold it!" Luffy demanded, thrusting it into his hands.

"I doubt I'm a carrier Lu'-" Ace froze, stunned when it began to glow, the crystal sparkling brightly.

Dadan quickly snatched it off him and threw it back at Makino. "Keep your hands on it girl! Before it registers Ace!"

Makino froze at that before doing exactly what Dadan said.

"Ace don't go near any of those crystals again. The crystals given out by the priestess' can somehow 'lock on' to a carrier and alert the faithful that there is a new carrier around. This one is thankfully Makino's so it can't lock on to you if you don't hold it for too long." Dadan said gruffly. "Garp would kill me if some faithful came looking here. If they saw where you lived they'd drag you off to go live at their temple!"

"Why?" Luffy frowned, confused.

"Those bloody priestess' believe a carrier should be treated as a gift from God, meaning in their eyes Ace would be a precious, fragile gem in need of gentle handling and locked inside a gilded cage for his own protection. They'd drag Ace off and lock him in their temple if they saw he lived in the forest and hunted food for himself and played in the dirt and muck all day and was at risk of getting hurt all the time." Dadan snapped.

"But I can look after myself. I've been doing it since I was four." Ace frowned.

"Those idiots don't see it that way."

"But Ace is so strong!" Luffy protested.

"They've got the Marines in their pockets because they give away carriers to the Celestial Dragons." Dadan grunted back, ignoring Makino's gasp. "Of course no one ever says it but that's what they'd do. Any precious carrier living in their temple doesn't choose who they marry. Most are given to nobles and Celestial Dragon's the continue on their bloodline and the Celestial Dragon's have the Marines under their boots. If Ace somehow managed to get away from those fanatics they'd have the Marines get him instead."

"Okay... Don't go near those crystals. Got it." Ace grumbled.

"You can go near the ones the faithful ain't using. The shady dealers always sell them without the 'lock on' feature." Dadan offered with a shrug. "Never guessed you'd be a carrier though Ace."

"It's not like I'll ever be carrying a child anyway so there's no point mentioning it." Ace replied.

"Why not?" Luffy tilted his head.

"I'm gonna be a pirate. Ain't like I'd settle down and start a family. I'm gonna sail the seas and become a pirate remember?"

Ace also knew no one would love him enough that way either. He had Luffy and he had Sabo once, that should be both enough.

"Shishishi, we both are! I'm gonna beat you to One Piece and become the pirate King!"

"I'd like to see you try you crybaby!"

(Okay, so the storyline remains pretty much the same. Ace leaves at seventeen, gets kidnapped by the Whitebeards pirates and they one day become his family. Marco and Ace develop a romantic relationship and so we get to see what happens during these stages and yes this is gonna be a mpreg Ace story! I'm excited to write it :3)