Sure thing! This is a S4 fix-it, inspired by this post by @fishfingersandscarves -- aka, I saw that post and immediately word-vomited into a document so I could come back to it later if I wanted, lmao.
The basic premise is: Teddy is early/mid-teens-ish, and (via a sequence of events I have kind of thought about but tbh mostly amount to “this show doesn’t give a shit about the rules of magic so neither will I”) horomances his way into the timeline in an attempt to save Quentin’s life, but overshoots and lands further back than he intended, ie, smack dab in the middle of “Toddler Monster Takes Eliot on a Murder Spree”. He decides that it’s better to wait it out instead of risking anything by trying to jump forward, so when Quentin (naturally) wants to send him back, Teddy lies and says his horomancy-box-thing is broken, and he can’t go anywhere until it’s fixed.
This almost works-- except that Julia, being Julia, ofc has his number and knows that he’s lying. Teddy won’t tell her why he’s messing with the timeline, but he does convince her that it’s serious enough not to immediately kick him back where he came from, so she agrees not to rat him out to Q on the condition that Teddy let her chaperone him and, hopefully, get him back home sooner rather than later.
It’s mostly Julia and Teddy, with a side-helping of Julia and Quentin, and probably just enough queliot to earn a “Quentin Coldwater/Eliot Waugh (Background)” tag on AO3. Here’s a snippet of the most coherent bit I have so far:
“You’re sneaky,” she observes. “Where’d you get that from? I know it wasn’t from Q.”
Teddy squirms, exactly the same way Q used to when he was thirteen and knew the answer, but didn’t want to say it. It’s almost spooky, how uncanny it is.
If Julia learned anything from middle school, it was that sometimes she just had to approach a problem from a different angle. “He met your mom in Fillory?” she asks.
“She died,” he answers, very even, very rote, like it’s a question he gets a lot. “When I was little.”
“Oh,” Julia says. “I’m really sorry to hear that.”
Teddy shrugs. He’s not looking at her, scraping down the sides of his ice cream bowl instead.
“So it was just you and your dad?” Julia asks, and that— there. His eyes dart away, toward the door.
And Julia… knew. Or, she suspected, anyway. She’s not surprised, because she knows, okay? She knows what it looks like when Q is in love. But she’d always assumed it was Brakebills, before Alice, or— after Alice, or Fillory, or whatever, wherever, whenever. She’d thought it was just another texture of the part of his life she missed out on, that he locked her out of, that she cut herself off from, however you want to frame it.
She guesses she was right. Just way more right than she ever thought she could be, with Q.