After his fifth year, James Potter realised how much of a bullies he and the other Marauders are and decides to do better from now on. After Sirius appeared one night on the Potters' doorsteps and begged for a place to stay (of course his parents agreed, how could they not?) Prongs and Padfoot started discussing James' decision and after the initial reluctance, Sirius agreed with it. After all, he can't even remember when was the last time he talked with Reggie, without them insulting each other.
When they get back for their sixth year, they talk with Moony and Wormtail and in the end, the Marauders come to the conclusion that the best way to show their decision, would be to finally end their 'war' with Sni-no, with Snape and form at least a truce of sorts.
Then they decided that since James saved Snape's life last term, he should be the one to apologise. When their approach not only doesn't work, but makes Snape's hate them even more now, because of the new rumours about his and James' sexual orientation. James explains the situation and asks for help his aunt Dorea, a Slytherin through and through. In her usual snark, she first berates his uncouth behaviour and then proceed to explain that James has to stop expecting Snape to act like a Gryffindor. Moreover, maybe he should find at least one good way of utilising all those years of 'pranks'. In the end the suggestion works and maybe Snape and Marauders will never be friends, but they gained a kind of understanding of each other.
Years later when Harry Potter is once again shot with the killing curse and gets to talk with the vision of Dumbledore on the empty King's Cross station. The late Headmaster gives him a choice to either go on to the ‘next great adventure’ or go back and finish the fight against Voldemort.
Harry gets the idea that maybe getting back isn't limited to just the current time, maybe he was just given an option to save all those good people that has died along the way.
So he goes back as far as he can. Moments later eleven years old Rose Potter, the Girl Who Lived wakes up. After getting used to the gender change, Rose realises that isn't the only different thing about her new reality. It is strange not to be hated by Snape and she is quite sure that she would remember meeting Fleur before Hogwarts.