Dryad!Lily leaves her dryad son to the trees, who take the literal sapling to Severus Snape’s door. A vow is struck: Severus will guard the sapling and help it to grow, and the trees will protect him from both Voldemort’s remaining followers and Albus Dumbledore’s (trees don’t give a fuck about politics or who wins a fricking war, they’re loyal to Lily’s last wish before she gave herself up to stop Voldemort because she was an old, powerful dryad). Remus, during an expedition to Cokeworth to search for Severus, stumbles upon a tree that hints after questions into Harry’s health that Severus is protecting the boy. After hiding this new knowledge from Albus using the werewolf’s natural mental possessiveness, Remus runs off to Severus’ tree circus. End result: Remus/Severus, asexual!Harry, confused literally everyone else, Voldemort and Albus Dumbledore’s plans foiled completely, and canon divergence to the max.
Lily Potter is able to pass as a witch. Her magic, while powerful for a dryad, resembles a witch’s enough for her to snag a Hogwarts letter to look after Severus as he leaves the neighborhood where she can protect him. She likes the boy; he sits under her tree and does his homework and doesn’t bother the other children despite how much they bother him. His mother is a witch, so he knows about magic, and he is wiling away the days until Hogwarts acknowledges him. She approaches him as a muggleborn, curious about this boy, and by the time he’s eleven she doesn’t want to be without him.
Going to Hogwarts isn’t too difficult for her; she’s an old dryad, older than all the younger dryads in the area, so once she gets to Hogwarts, weakened as she is by the distance from her tree, it’s easy to convince the Whomping Willow, as it’s known to the students, to accept her care in exchange for some of its life.
The years go by. She has a good laugh when she realizes that young James Potter, brash and laughing, has a massive crush on her, Lily Evans, immortal tree spirit and chosen protector of James Potter’s antagonism. Going to his side seems to stop the bullying, though, and he isn’t that bad once she gets him used to the idea that Sev isn’t interested in her in the least.
(They’re sitting in Gryffindor Tower and James is staring at her. “He’s gay, James.” She says calmly, and the page of her book turns with a controlled bit of magic. James sputters. “He doesn’t like me like that, James. We just lived in the same town before Hogwarts and became good friends, and he’s lonely, James. You know the Slytherins can’t stand him because he’s my friend.”)
So Severus stays her friend through the ‘mudblood’ slur because she’s a tree, darling, it isn’t blood in her veins, and James is her partner until the end of school, at which point they are married. She wonders how she’s going to break the fact that all of her children will be saplings instead of fetuses to James when he decides he’d like to bring up children with her. (How is she going to break the fact that they’ve already got a child, growing in the backyard beside her garden?)
And then there’s Voldemort sneaking through her house like a creepy stalker and killing James to get to her because their child, whom James didn’t know existed yet, was destined to kill the old, stupid man. The Killing Curse gets trees and spirits just as easily as humans, so she whispers to the trees and out comes her old friend, unnamed because she doesn’t bother with humans and dryads have no need for names, responds, taking her little sapling away to Severus, the boy she still meets every week and watches out for and cares for.
As she finishes, Voldemort steals out of her house, angry and vibrating with his wizard’s magic and believing himself better than her and she is laughing. This is a boy, and she is a tree’s guardian and she will destroy anything that threatens her budding forest or her lonely little boy.
She is an old tree, and this final transformation will show her age completely; by taking this rotten beast with her, she will exponentially increase her final form’s power. The final transformation into her tree body that steals her life and leaves it as a powerful magical boost to anything nearby.
Moments later, there is nothing in Lily Potter’s backyard but an aged tree flowering even in the winter air and a pile of ash where the branches and roots had reached for a man already dying and torn him apart.