Well after reading the sixth HP/SG-1 crossover that have Harry going back in time and becoming Daniel Jackson so have I decided to post this challenge.

Harry defeats Voldemort right after he graduates from Hogwarts. HBP never happened, or if it did so did Dumbledore not die.

Once Voldemort is gone does the Ministry tell anyone that will listen that Harry has been setting himself up as a Dark Lord and Harry just leaves.

He goes back in time and becomes Jack O'Neill. He finds that Jack O'Neill and his parents die in
a car crash when Jack was 15 and he then takes Jacks place as he had no other living relatives.

The story should take place in Season 9 or 10 of SG-1 and have Jack struggle with himself about involving wizards with the SGC to counter act the power of the Priors/Adria.

The wizards cant solve all the SGCs problems and they will have large problems with accepting aliens, the stargate and such.

Bonus points if at one point had the wizarding world worshipped the Goa'uld and belived that the first magic useds had been created by them.

Wizards and Witches can not be Ancients, they should be an experiment done
by the Goa'uld.

 

But why wouldn't jack have used his magic before? Like when being tortured by Ba'al?

 

In part because he had no wand and the other reason was that he knew that if he used his magic and the Goa'uld learned of it would they not stop until one of them took him as their host, then would they learn of the wizarding world and nothing would stop them from coming to earth, not even the Asgard.

 

Forgot about Jacks brief time as a Tok'ra but as you pointed out so would he not have had any chance to tell the rest of the Tok'ra...