HP x Haikyuu!
Fem!Harry
Harry x Asahi Azumane
(Asahi is first name)
Harry is recruited as a manager alongside Yachi.
Asahi's sweet/gentle personality would be just what a war torn Harry needs. (And someone who isn't loud)
HP canon events squeezed together. 1st and 2nd year together for year one, 3rd and 4th, and then 5th and 6th, 7th could be meshed in with 6th or mainly taken out if the horcuxes didn't exist outside of the diary and Harry's scar (and voldemort's current wraith/soul form).
That would mean the war more or less happened when she was 15/16. That would make her more or less a freshman/junior when she moves away to Japan.
Maybe by the time canon begins she would be a second year manager (to have her age more accurate).
Another Idea!
Harry is somehow related to the Ukai's (Keishin is her uncle, or is at least called uncle much to his displeasure. Because at most his is just a decade older). The retured coach Ukai would be her great-grandfather if Keishin is her uncle.
This Harry moves to japan to spend time with family other than the Dursleys.
She goes to Kurosuno but instead of taking a school sport (let's say the school doesn't have a team or club for this) she takes dance lessons at a local center/studio (I think dance would fit her tiny stature).
One day (after Keishin agrees to coach), Harry shows up at the gym in the school uniform (I think I still want her to be a second year), looking for Keishin. Like she opens the doors looks around (she ignores the attention she gains) and when she spots Keishin calls out 'Uncle' and quickly walks over to him (after slipping off her shoes). Keishin is surprised and says "Rī! What are you doing her? Shouldn't you have left by now?" Harry then shoves a box into his face, and says something along the lines 'I made too much for lunch, and you know food isn't allowed in the studio, so I thought you might like it instead of wasting it'. Keishin just nods and agreees he'll take care of it. They then go on to discuss who will be cooking dinner tonight, Keishin agrees that if Rī is too tired when she returns that he'll cook.
(Wait doesn't Keishin's mother still live at the shop?)
Harry then turns, finally notices the rest of the club, nods at them and leaves. They all turn to Keishin then (who sets down the bento) and demand to know who the pretty girl was, and how she was related to him.
"Ah. That was my niece Rī, she is currently living at the shop with me while she is going here. You second years might know her, since she is in some of your classes I think."
The team then all turn to the second years, and they all look stumped, but finally (and surprisingly) Tanaka exclaims! That yes he knows her! Or has at least seen her! But that Rī is very quiet and well he only remembers her because she might actually have been shorter than Noya!
The club (and canon) go back to normal and plays forward. Rī starts to make more appearances (mostly for the same reason, let's say her dance lessons have just multiplied or started in ernest), the boys (and girls) get used to seeing her on occasions though they still haven't heard her talk to them (all she does is that nod of recognition/goodbye).
It is finally Asahi who speaks to her, since he was going to be late to club and accidentally bumps into her on the way (he knocks her over and more or less falls on top of her in that cliché fashion), he quickly apologizes and helps her up. Harry reassures him that she is fine, and reminds him that he will be late at this rate, so she walks with him as his excuse for being tardy.
As they talk, Asahi is surprised to actually learn that Rī knows a whole lot about volleyball, and even more about how the team is doing (like the games and who plays what). Though he realizes that if she is related to Ukai and lives with one then it should be obvious that she would be knowledgeable on the subject.
The two to the surprise of the others are talking enthusiastically to each other as they enter, and both walk away from their unplanned meeting more or less as friends (most thought that Asahi's looks would have scared her away).
The two now tend to run into each other more often.