TL;DR - one of my players wants to retcon her backstory, to say that she was really a Pixie,.who fell in love with with a human and struck a bargain with Baba Yaga, to turn her into a human so she could be with the one she loved.

Any ideas on how I could work this into my game, while still keeping things interesting for the other players, who will each get their turn to tell their stories through our 2019 campaign?

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Through out our 2018 campaign, she played a Druid (she wanted a pet wolf so I let her have that)

Now, I am asking my group for ideas about where they would like to go in terms of adventure and character development, when we start back up in January for our 2019 campaogn.

The player of the Druid character, said she would like to take her story in an unexpected direction and when I asked her what she meant by unexpected, she said that she would like to be a Pixie who fell in love with a human mortal, and so struck a deal with the the evil entity; Baba Yaga, so that she could become a human and be with the one she loved.

Baba Yaga granted her request but instead of just making her a human, she used a powerful reserectiom spell on both her and the human that she loved - that basically killed the Pixie that she was and killed the human boy she loved, and then resurected her into the unborn child of a Druid woman who had recently miscarried and her love into the body of a wolf puppy.

So she grew up as a human Druid, not knowing of the bargain that her former Pixie self struck with Baba Yaga, or that her pet wolf was really her former lover.

In her arrogance and willingness to corrupt the pure wishes of Pixies heart, Baba Yaga made a mistake. Her spell didn't totally erase the memories of her past life as a Pixie and now that she has become a fully fledged Circle of the moon Druid, her intense connection to nature is starting to unlock slivers of her past and memories of life that she should have lived.

She wants to play through the exploration those emerging memories and throughout 2019, rediscover the truth of her existance as Pixie and the bargain she struck and the terrible price that she has paid for the sake of love.

Any ideas on how I could work this into my game, while still keeping things interesting for the other players, who will each get their turn to tell their stories through our 2019 campaign?

Coven of hags in service of Baba Yaga are aware of the pixie reawakening her memories. They make an effort to please Baba Yaga by killing/capturing the PC or taking away her memories permanently. Luring the party with standard hag plots like kidnappings kids or turning livestock into flesh hungry monsters. The hags can become recurring villains and hound the party over several individual quest lines. 

I always try to dig by asking why until it stops making sense.

• Why is your player in the party and not with the one she loves? Sounds like she already had her adventure and should be enjoying the spoils.
  • Did he or she die?
    • How and why did the lover die?
  • Did Baba Yaga perhaps take the character's lover as payment? Rescue mission :)
• Why did Baba Yaga help her?
  • Baba Yaga doesn't just do things like this. She wanted something in return. Maybe the player has to find an item to pay Baba Yaga back within a certain time limit. Put that interesting magic item into another part of your campaign, maybe make it a somewhat evil item. Don't do this if you hate moral grey area decisions, but requiring this character to give Baba Yaga a powerful evil item in exchange for keeping her life is certainly controversial. 
• Why is it permanent?
  • Make it so the character has to regularly perform a ritual or reasonable sacrifice of some kind to keep the transformation going.

...Keep going! I always find digging and digging like a little kid asking too many questions always takes you to interesting places. Bonus points to do this with your player- it makes them so much more connected to things.

Well from what she wrote, she didn't just run off to Baba Yaga.  She first asked her own own people for help.  Those amongst them who were powerful enough to do what she asked refused because they couldn't understand why she would fall in love with a human boy.

Why a human boy might fall in love with her, they could understand that, but why she would fall in love with him was something they couldn't really grasp.

So other Pixies and even other Fey wouldn't help her but her love for this human boy was not just a passing curiosity, it's something she felt in the very core of her being.

So she was sad, feeling lonely and in a bad place when she came across Baba Yaga; who acted like a kind old woman, listening to her, making I seem like she understood and sympathised.

So the two would meet a every now and then and Baba Yaga would listen to the Pixie and her sorrow.  Then one day, after she had befriended her, she offered to help.

Like "I know how you feel and we have known each other for a while now and become good friends.  I hate seeing you in such a bad place, so seeing as we are friends - I can use my magic to help you.  I can turn you into a human and ensure that you and your beloved spend the rest of your days together"

She requested no payment for her services - insisting that she was a friend helping out a friend in need.

However; it was a trick and although Baba Yaga was true to her word, she was not true to the spirit of her word.  In a strict interpretation though, she did exactly what she promised to do.  

She turned the pixie into a human and made sure that her and the one she loved would spend the rest of their days together.

How she fulfilled that promise though was a warped, twisted idea of what it should have been.

So we now have a Pixie come Druid in the body of a child that should not have been born and a family who morns the loss of a beloved son (now in the body of a pet wolf).

In fulfilling this Pixies wish, Baba Yaga might seem to have getting nothing from it but she has sown the seeds of discord and sooner or later, she will reap the harvest of that discord.

Because as you said, Baba Yaga does not do anything without payment.