So I've read a fanfiction where Salazar saved the potter and used a potion to turn into Harry and took his place as the boy who lived. This idea was inspired by it, its on fanfiction.net
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Salazar Slytherin became immortal and watched over the wizarding world for centuries. However he became immortal at a price, he will have to be turned into a snake.
(Salazar is a good guy, not a bad guy in this idea)
When he found out what his descendant done he was disgusted. He somehow finds out about the prophecy and decides to teach Harry Potter.
He tracks him down and becomes his familiar and teaches Harry all about magic and the wizarding world.
Or
What if the Snake in the zoo that Harry talked to was in fact Salazar Slytherin.
He decides to stay with Harry and teach him magic.