How about Rick Riordan's Percy Jackson series as a prompt for like Marco/Ace?
I probably took this the wrong way, but here you go friendo.
Marco isn’t sure of what to make of the new kid. He’s a spit-fire, enough so that Shanks laughs when the kid blows his top sipping his drinks with a forlorn expression of a man that can’t have alcohol, but he’s not rude. It’s easy to think that someone so touchy and bull headed would be rude, but the kid has manners. Which is in his favor as the days pass and he stays in the Whitebeard cabin.
The Gods know that they only have a few weeks to claim their children before Whitebeard takes a liking to them and claims them for his own. It’s how they got Haruta after all, one of the other Gods just a touch too slow to claim their child. Which means that Marco knows their father is looking to call Ace his own.
Ace hadn’t expressed his opinion on the matter either way, just kept his mouth shut and hovered close to two of the boys that had been claimed by Dragon, one was actually the God’s son and the other was a minor deity of revolution. Both of whom declared that he could come and live with them if he wasn’t claimed.
Which of course had let to an argument with Mihawk’s heir, not his son just a boy that he had picked up somewhere, and one of Bell-Marie’s daughters about how they couldn’t take Ace from the Whitebeard Cabin.
Marco didn’t understand why they thought the rules were so strict. It wasn’t like Izo didn’t spend half his time fliting in and out of other cabins as he pleased. But he would let them figure that out for themselves.
“I’m thinking Shanks or Big Mom,” Izo states flouncing to sit by Marco. “For Ace’s parent.”
“Mother was human,” Marco reminds him. “He talks about her quite about, which means it can’t be Big Mom.”
“Fine, Doflamingo then.”
“We aren’t related,” Law states kicking at the foot Izo has on his seat. “I would know that much. Cora-san makes sure that I’m kept up to date on Doffy’s children.”
“We don’t usually have this much trouble pinning people down,” Marco muses.
“Not since Dragon’s kid came along,” Thatch adds. “I mean who would expect that to be Dragon’s?”
Luffy, that’s the name of Dragon’s son, is the happiest demigod that Marco has met in a very long time and Marco has met many demigods. Especially for one related to Dragon.
“Maybe he’s like Marco,” Haruta offers.
Marco tips his head in acknowledgement, after all there were other mythical creatures in the camp, even if he was the only Phoenix.
“Oh,” Marco looks up as Ace starts to glow. “I wonder if Pops got impatient.”
Ace is too busy frowning at his glowing hands to see the initial appearance of the symbol over his head, though he looks up quickly enough when the rest of the area falls into a hush.
“Shitty asshole,” Ace hisses marching off, symbol still of his head.
Marco raises an eyebrow, wasn’t that interesting? The new little spit-fire was Roger’s. He didn’t think the man had children, what had made Ace’s mother so special, he wondered.