So, imagine Sirius got ahold of the Daily Prophet the summer before Female Harry (hereafter referred to as Ianthe) started first year - right about the time that the Prophet was all fangirly over the Girl-Who-Lived returning to the magical world after ten years exile with Muggles. Cue breakout early, because how much time has passed and what Muggles? Imagine, also, that Sirius actually ran true to the background JK Rowling gave the Marauders: brilliant, powerful and creative wizards that could face Voldemort and survive three times, who invented whole new spells by their OWLs and managed something as students that literally two other people in the entire series demonstrated.
So, Sirius escapes Azkaban and does something smart - he takes himself off to Gringotts, requests Sanctuary, takes a few weeks (and a lot of healing potions) to get his head on straight after years of Dementors, and to plan – because a Marauder always has a plan (not necessarily a good plan, see Halloween attack on the Fat Lady as exhibit A).
Problem: wanted men don’t get custody of their sweet goddaughter. Solution: get a trial.
Problem: The Ministry is full of morons and Fudge is Moron‑in‑Chief. Solution: blackmail key members of the Wizengamot into clearing way. Freedom achieved.
Now let's imagine that Sirius, aided by Remus, are unimpressed by the Philosopher Stone, or a reasonable facsimile, being placed in school full of children - curious ones. And also the Ministry. And Dumbledore – especially his secrets. And the Dursleys. And Ianthe’s lack of knowledge of her family and rank. And any number of other things.
Cue the Marauders doing…marauder things. Like screwing with people’s plans and destroying Horcruxes and cockblocking Dumbledore’s Greater Good. Ianthe comes by trouble very naturally. She also learns her Marauder lessons well. Pranking and politicking have a surprising amount in common if you do them right. Hermione is horrified, awed, and impressed. Lucius Malfoy wants to know what the fuck happened to his controlled, well‑ ordered world. Dumbledore is furious that his chess pieces have staged a coup d’etat.
Six years later, Voldemort is still a wraith, the horcruxes are gone, and Ianthe and Sirius are attacked by several Death Eaters and He Who Has No Body.
Problem: wraiths can’t be killed. Solution: kill him while possessing a body. Problem: he’s trying to posses Sirius’s goddaughter. Solution: give him someone else to possess. Problem: you need to kill the spirit trying to possess you before he actually does, in a way he can’t escape.
Ianthe does not approve.
In another time and place Ianthe saved her godfather by meddling with time and by breaking two international laws. In this time and place? Ianthe offers her magic, soul and life to change a single moment in time, not only to save her godfather’s life, but to change his past.
The part where Merlin shows up and dumps her in the 1970s? Not part of her plan. Fortunately, pranksters are adaptable.