Idea:
After some crazy adventure fighting Opal, Artemis & Co's lives are drastically changed. Holly is the same (or maybe younger), but Artemis has turned into an elf. Reason being: time stream malfunction, some kind of Opal scheme, or something happened, and Artemis' mind was implanted in a brain-dead elf's body (a brain-dead elf who looks like an elfin version of Artemis) after his own body died (thanks to Opal). Also, Beckett Fowl is murdered by Opal, which makes Artemis angry enough to execute Opal once and for all, and Minerva Paradizo is spun back to Myles' age.

Afterwards (not shown in hypothetical book): Artemis marries Holly Short, Myles Fowl marries Minerva Paradizo. As an elf, Artemis is now living underground with the People. Myles and Minerva inherit Fowl Manor after (or maybe not, who knows?) Artemis I and Angeline Fowl's deaths. OK. Background set. On with the scene:

Twenty years later...

"Beckett Julius Fowl, stop that this instant!" The short, red-haired elf exclaimed. Two adult elves stood hovered in the dining room of Fowl Manor, with a third younger elf who was swinging to and fro on several large diamond chandeliers. It was to this third elf that Holly (the speaker) addressed her complaint.
"Aw, Ma..." the young elf whined.
"Don't you 'Aw, Ma' me, Beckett," Holly replied sternly. "Those chandeliers are expensive. And heavy. You wouldn't want to go around destroying your uncle's furniture, would you?"
"They're not that expensive, are they, Dad?"
"Two hundred thousand euros for the lot," Artemis Fowl II replied. "Without insurance." But his mind wasn't on the conversation with his wayward youngster. Upstairs, the door burst open, and Myles Fowl, several feet taller and many more inches wider than when we last saw him, sprung down the stairs, giddy as a dwarf in a gold mine. "It's a boy!" He exclaimed as he plucked his airborne nephew from the nearest chandelier and nearly strangled him in a rapturous bear hug.
"Awrk!" Beckett protested from within the confines of his uncle's embrace. "You're choking me, Uncle!"
"Congratulations, Myles," Artemis grinned, ignoring his son's discomfort.
"How's Minerva?" Holly added.
"Fine, fine, they're both doing fine," Myles replied, releasing a very relieved Beckett from his stranglehold. Scowling, Beckett adjusted his collar and smoothed his crumpled test flight wings-a gift from his friend, Fallina, whose famous father, the genius Foaly, was still hard at work patenting new inventions every other second.
"Can we see them?" Artemis asked.
"Of course!" Beaming, Myles led the way up the stairs as the trio followed, opting to fly instead of walk. The bodyguard, a younger, thinner version of the retired Domovoi Butler-who was currently traveling in Mount Fiji, training new bodyguards with his famous wrestling sister Juliet-stood protectively outside the door. When he saw Myles in the lead, he stepped aside to let the party pass.
Inside what used to be Angeline Fowl's room, a very tired but happy Minerva Paradizo Fowl lay on the king-sized bed, cradling a bundle in her arms. She looked up to see her husband and in-laws walk in, and welcomed them with a weary smile.
"Artemis. Holly. It's good to see you again."
"Same to you, Minerva." Holly descended gently beside the head of the bed. "How are you holding up?"
"Quite well, actually. Look." With infinite tenderness, Minerva proudly showed off her new bundle to the former LEPrecon elf. Beckett crowded in beside his mother for a peek.
"Is that it? He looks a bit...tiny."
Holly snorted. "No tinier than you or your sisters were when you were this age, Beckett."
"Yeah, but we're elves. This one's a human."
Minerva smiled, her recent ordeal mellowing her out considerably. "Humans start out small too, Beckett. Didn't you know that?"
Beckett shrugged and hovered a few feet off the ground. "I guess so."
By this time, Artemis, too, had come around for a peek at his new nephew. "He's a handsome one, alright," Artemis pronounced solemnly. "What are you going to name him?"
Myles and Minerva eyed each other.
"Well, originally we were going to name him Beckett..." Myles began,
"...but two Beckett Fowls in one family would just be too confusing. Even if they were of different species." Minerva finished for him.
"So...?" Holly prompted, raising an eyebrow.
"So we're going to name him Artemis. Artemis Fowl III, to continue the family tradition." Myles beamed at his older brother.
"Hey, cool, Dad. They're going to name 'im after you." Beckett peered at his little cousin with more interest than before. "He kinda even looks like you. Except he's more wrinkly. And blond."
Artemis was touched at the honor given him by this naming. Not knowing what to say in response, he placed a hand on Holly's shoulder and smiled back at his brother and Minerva.
"We were hoping you and Holly would agree to be his godparents," Minerva said.
"Of course," Holly responded instantly.
Artemis nodded, musing. "You do realize that young Artemis III will be the first human since the Frond dynasty to have a fairy godmother, don't you? And perhaps the first human ever to have a fairy godfather."
"That makes him one special-and influential-little boy," Holly agreed. "Let's hope he doesn't turn out to be just like his namesake."
"Why, Holly, I'm wounded," Artemis feigned dismay. "I thought—after twenty-five years and three children—we'd gotten over that unpleasant speed bump from the past."
Holly laughed. "Gotten over doesn't mean forgotten, Artemis. But don't worry. I'm little Artemis's godparent too. I'll make sure he doesn't formulate any fairy-kidnapping schemes before he hits puberty."
Beckett, floating several feet above the adults, rolled his eyes at his parents' good-humored teasing. "This Artemis won't have to formulate any fairy-kidnapping schemes before he hits puberty," he pointed out. "He'll know all there is to know about us."
"That's right," Artemis II replied. "He will be surrounded by fairy magic his whole, long life."
Holly grinned. "It's going to be one heck of a life."