Ok, I own nothing, Buffy is Joss' and Stargate is someone else's, (can't remember who's), and I'm just playing with 'em.
The Powers That Be gathered together to discuss the outcome of the Harvest. They were pleased that the Summers Slayer had stopped the Master from freeing himself, they were also impressed by the Harris boys actions, staking his best friend to save a girl he heartily disliked, just because she was human. They knew he would go far.
But the boy who was staked, he too was supposed to be a Champion, until Summers dropped the ball and allowed him to be captured by Darla, and turned by the Master.
"Do we give the McNally child a chance to prove himself? The Harris child shows promise now. Do we change things to allow the McNally child a second chance?" one of the Powers asked the Others.
"It is true his promise and potential has not been utilised. However, it would be a disaster to send him back to the Hellmouth. Where can we send him?" asked another of the Powers.
Then the Oracles appeared, "He is to go to another planet, where he will fight the imposter's of the Gods."
"He will learn to be a Warrior amongst the descendants of Slaves," insisted the other Oracle, "this we have forseen."
"What planet is this to be done on?" asked one of the Powers curiously.
"We have seen Abydos," the Oracles replied together.
"Then Jesse McNally shall go to Abydos," all of the Powers intoned. And they started to reincarnate Jesse McNally from the ashes of the damned.
By the time the process was complete, a frightened 15 year old youth lay on the floor wondering where he was.
"Jesse McNally, you are being given something very rare and special," said the female Oracle. "You are being given a second chance at life."
"Do not waste it," the Powers said as one. And then the 'room' flashed white, and Jesse disappeared to Abydos.
When the great flash of white ended, Jesse looked around, and found himself in what resembled a mountain village.
"Man, this sucks," said Jesse under his breath, and he rubbed his left hand over his neck, and found a scar. "What the hell's that?" And then he remembered everything that had happened over the last few days in Sunnydale. Including being staked. By his best friend.
He was found 15 minutes, huddled in the corner of the building, hugging his knees and crying. After much coaxing, he was taken to see the leader of the settlement, Kasuf.
At this time, Daniel Jackson was still living on Abydos, the blight known as the Goa'uld had not stolen his Sha're from him yet, so he had a chance to talk to Jesse. But Jesse had taken the wrong end of the stick from the Powers warning, and it took three weeks to draw him out of his shell. Which coincided nicely with the return of Jack O'Neill. And heralded the return of the Goa'uld to Abydos.
During the attack against the defenders of Abydos, a most peculiar thing happened to Jesse, he knew what they were. He knew how to fight them, and what their weaknesses were, but his body was weak and untrained, so he did the next best thing. He got the children out of the way. And swore that he would kill these aliens that took over your body much like a demon had previously done to him.
Over the next few weeks, Jesse was either with Skaara's friends and guarding the chaapa'ai, or involved in some form of weapons or fitness training, or he was studying the galactic address book that Daniel had found. When he wasn't doing any of these, he was usually asleep.
It took 6 months for Jesse to talk another 5 Abydonian's into forming a strike team with him, and a further 3 months before they were ready to go out on their first hit and run strike. But the Powers were watching. And they approved.