Challenge Harry Potter is Salazar Slytherin
So Harry is at Hogwarts, he disappears in a flash of light. He meets Godric Gryffondor just after being treated.
Years pass, godric and Harry now Salazar meet two youg women with big projects. They appear to Rowena Ravenclaw (RR) and Helga hufflepuff (HH). Their idea is to build a castle near Hogsmeade to teach the next generation and so on.
During the construction of the castle Salazar falls in love with Helga (or Rowena according to your desire) and tells her a little (lot) of his life. The relationship grows more and more he opens. He tells her that this time is not really his and he comes from the end of the secon millennium (the choice is yours but the date must be after the second year). Without doubt that the woman, he loves, has the same kind of story. Because she also cames from the same date oh him.
You can make him back a few days after his leaving but he will not aged. As for his wife, she can return the next day of his disappearance or ensure that this never happened.
The year following his brief more or less along the canon (book, movie or both like Peeves) according to yours needs.
At the next sorting the tri hat in his song must know that it knows, they came back (the two founders) without students and faculty's knowledge.
Harry could tells that he is really to Hermione (when she didn't choose for the time travel) except Ron who is anti-Slytherin (must be bashing).
You can show (write) in flashback pieces of his life for a better understanding.
He'll take a turn for Gringott has recovered some property (jewelry, thing, etc...) in his vault and that of his wife through the authorization of marriage.
I am more for a Harry Potter/Susan Bones/Salazar Slytherin/Helga Hufflepuff fanfiction. But you can choose another woman as Hermione, Daphne a woman of his age (more or less 3) except Ginny (this is why I hate the epilogue in DH).
To avoid searching in all couple (like Harry/Susan or Hermione...) placed it in Harry Potter and Salazar Slytherin categorie.
If you need information mp me for those whose want to do this challenge.
Sorry if my grammar is bad so don't take my faults in account.