HP x Haikyuu!
Fem!Harry is Kiyoko Shimizu
Harry x Asahi Azumane
One of the trivia facts is that she played a sport prior to becoming the Karasuno volleyball club manager, but the sport is unknown... Quidditch!
Kiyoko - Pure Child
Shimizu - Clear Water
Capricorn - 17 years old (3rd year high school student)
DoB - January 6th
Harry Potter
Leo - 17 years old at end of series
DoB - July 31st
Asahi - Morning Sun
Azumane - East Peak
Capricorn - 17 years old
DoB - January 1st
Would have to be 15 when starting at Karasuno (a 1st year), school year starts in April. So after The Goblet of Fire, Sirius takes Harry (secretly) to Gringotts and does some financial stuff, and takes them to Japan (where either the Blacks or the Potters had a countryside house, near Karasuno). They go through the goblins and the Japanese ministry and through this Sirius is cleared (in Japan at least) and they can now live as they please, though they do legally/magically change their names. Harry to Kiyoko Shimizu, and Sirius to the Japanese equivalent (Tenōkami- The Dog Star) But we'll just keep him as Sirius, cause I don't know how often the others would react with him). So Sirius would be Kuro Sirius basically.
Actually make his name Shimizu Sirius (Tenōkami).
Sirius is still Harry's godfather, but legally they have him marked down as her godfather, uncle, and guardian.
And basically since Harry has now been removed from the British Wizarding World all the events that happen from the end of the 4th book forward never happen, she will no longer be involved in the war, and the British Wizarding World will be ignored from henceforth.
To get things started off, the house (more like small mansion or a rich lords/kings country villa), Kiyoko and Sirius move into is in the Miyagi Prefecture, and the Azumane family are their closest neighbors (I'd imagine there is quite some land around the villa, to make it self sustaining). Anyway, the two of them decide to go visit their neighbors and introduce themselves, and that is where Harry/Kiyoko and Asahi meet for the first time. (Asahi is awestruck with how pretty she is [I also think that Harry would have wanted to have an entirely new start so she did what ever she could to remove all her scared and improve her physical health (horcrux removed or never existed?), so she has basically unblemished skin] and due to his insecure/shy nature he bumbles through his introduction, but Harry fines him cute, and really she just wants a friend. And so friends they become over the next couple months before the new school year (in April, since by the time they arrive in Japan it is July/August, and Harry has already finished the school year, so she is just doing a lot of studying and test taking to catch up) begins. (Their guardians kind of think they're perfect for each other, even a reluctant Sirius, who became friends with the neighbors [and neighborhood by charming the elderly ladies]).
And so when the year begins, Asahi shows Kiyoko to Karasuno High School, and after the day is over and he learns that his friend/crush doesn't know what club to join he asks her to come with him to volleyball (their guardian/parents want them to walk home together, for Kiyoko/Harry's safety), so she does and some how she gets roped into being manager. And for once in a long time she's looking forward to her future.
I imagine Sirius decides to do freelance work for the Japanese ministry (he cant really leave Japan without trouble happening, since he is only deemed/proven innocent in Japan), or maybe he does some type of business in memorial of his friends, like a joke-shop or florist (magical flowers and plants), and he has become rather well liked and successful. (I like the flower shop idea, let's call it "Lily's Place" for now. And the shop could have both a magical and mundane side to it (I feel like a lot of Japanese shops would be like this, since I think the Japanese magicals would be a lot more blended in to normal society).