HPxOP
Fem!Harry
Harry & Chopper friendship (siblings? Or future lovers?)
MoD!Harry
Animagus!Harry (deer/doe)
Ate a Devil Fruit!Harry
Harry arrives on Drum Island sometime before the Strawhats, but after Wapol has been kicked out. Chopper finds her outside the castle, practically frozen to death, he brings her inside, where he and Docterine nurse her back to health. But then she escapes and tries to freeze herself to death again. When Chopper finds her (because he found her gone and panics), she begs to die, for them to just let her die, that she already should be dead, that she just wants to be/see her friends and family again, and why wont he just let her be. In an act of rare bravery (at the time, since this is before canon) Chopper somehow convinces her to come back in side, or to at least trying living a little longer, and he carries her back inside.
Time foes by, the two become close friends, and the three of them (Doctorine, Choppe, and Harry) become like family. Harry (whose name might be changed) learns medicine and all she can about the world she now lives in. She might also learn more magic, perhaps she had an extendable bag with her?
When she is finally coherent enough to realize Chopper isn't human, she doesn't even freak out, she just accepts it, which at the time is a great change from the normal reaction he gets.
And when Harry learns about the devil fruits
She researches for a Deer-Deer fruit, because why not? There's a human-human fruit and various other animals why not a deer one? She finds it eventually (but still before canon begins), like perhaps, when she is learning about the world she goes and travels to different islands all over (she still has magic), and she uses spells to help her find the fruit, and while she's at these islands, she gathers maps, books, and logposes among other novelties from the island.
Once she finds the fruit she doesn't eat it till she returns to Drum, where she inquires to Chopper if he would like for someone to be like him, if he would allow her to do so, because he means quite a lot to her now, and she doesn't want him to feel so lonely, not like she did. He is surprised but (secretly, because Chopper at this point still tries to deny praise) pleased, and agrees to let her eat the fruit.
I suppose Harry's magic isn't all powerful, like when she arrived in the One Piece world, her body was 'adjusted' to the new world's magic, so only her most commonly used spells and maybe a few others transferred over, also as the MoD, I feel like to make it simpler and to make her not have to worry about losing or breaking her wand, that her wand(s), cloak and the stone merged with her, and formed the Deathly Hollows mark on her skin, perhaps the lower middle of her back? And well I guess if she had the elder wand, maybe her magic won't be limited, but since it is now wandless, she has to work on relearning spells. There that sounds good!
So when she arrives on Drum, she is basically back to being 10/11 years old (since if i make her and Chopper a romantic pairing, I don't want to large of an age gap, will make a timeline later) both physically, emotionally,and power wise, but not mentally. This gives her time to grow and recover and learn before canon starts. So by the time canon events start she'll have had about 5 years to adapt to the world of One Piece, to relearn 5 years of magic (she took it slow and did one year of spells a year, of course learning wandless was harder then when she learned with a wand in school), gains a devil fruit that is almost more of an animagus form for her (though she does have her full human form, her half form[maybe two half forms? Like how Chopper has his giant(human) form and then his tiny (halfway?) form], and then her full form. But of course if Chopper was research how to make more forms for battle and defense, i feel Harry would be right there with him, she she probably doesn't use them as much because of magic), and knowledge from all over the seas, but with her main focus being on medicine (though her medicinal practice is more towards potions and poltices, like what she learned before, and not more like Choppers, who uses actual 'medicine' and 'modern' techniques.) and a little bit of navigation and cooking (since canon Harry cooked for the Dursleys), perhaps she even learned about plants, which would actually tie in nicely for both the cooking and medicine she learned.
Luffy would make her his ship's magician for obvious reasons, but she would help out in both the kitchen and medbay, probably help Nami determine some navigational stuff, and possibly help Franky and supply Usopp with odds and ends for his weapons. She would probably talk to Robin about the world and books that they've read, if she collected maps from her travels along with logposes, she would share then with Nami. And then she would entertain Luffy (especially) with magic. I am not sure about how she would interact with Zoro, like what would they have in common, cause I'm not really planning on giving her a weapon, perhaps she has gained some knowledge about swordsmanship, but doesn't carry or use one. Harry also wouldn't be much of a drinker since she would be physically around 15 in the story till the time skip. I'm sure I'll think of something. As for Brook, since Harry grew up the first time with Ghost, a skeleton won't bother her much, and also she's the MoD. No wait! With what Brook's devil fruit did allowing him to come back to life and how he had to live without his friends, they would get along swimmingly! Might make Chopper confusedly jealous (since he might not understand the feeling), because she would be spending more time with Brook then with him! Perfect!