season 7 for Buffy
season 5 for Stargate- before Daniel dies and before they meet Jonas Quinn

During the season 7 finale of Buffy instead of Spike's amulet's sun rays missing Willow some hit her
and her magic tried to repel some of the fatal beams and accidentally rebounded back onto Spike as another ray came her way- the sun rays, old amulet magic and Willow's white magic (which was a lot more power after the Scythe spell) crashed into each other forcefully, both energies fought and canceled each other out and consequently teleporting Willow and Spike to the 'Gate room in a...

Buffy/Firestarter crossover

Buffy: right after season 2's When She Was Bad

despite killing most of The Shop some of the members lived and came after Charlie McGee years and years later, Charlie managed to keep her only daughter Willow safe and protected from the bad guys.
but when Willow was just 2 years old the Shop captured Charlie and her daughter- barely managing to escape Charlie realised that she couldn't keep her daughter, who also had her Firestarting abilities with powerful Telekinesis, and made the most painful decision she had ever made, to take Willow to a new...

Buffy hasn't yet returned from LA at the end of season 2 after sending Angel to Hell.
Willow's mom and dad were about to attend another lecture in Florida but the plane they were in crashed and they died.
now Willow is sent to Jacob Carter- her uncle and meets her cousin Samantha Carter- the two cousins look nothing alike- but they both have a thirst for knowledge and learning.
now Willow has to get used to being in the care of two militants and their friends- especially this Murray guy who she doesn't think is human- maybe he's a demon!
Stargate after 'Last Stand' in season 5....

Instead of Willow staying at Sunnydale after season three she goes to the Air Force Academy and then joins a new SG2.
The three other members of SG2 are Lieutenant- now Captain Jennifer Hailey, Lieutenant Grogan and Major Louis Ferretti.
The two time lines of both shows are correct so Willow joins SG2 in 2005- four years after season 5 of Stargate (the everything after season 5's 'Last Stand' doesn't happen!)

Must not have:

The Air Force knowing about the Initiative- (if it happens at all)
The Scoobies finding out about Aliens

Must have:

Willow a...