A step to one side in time, a shift to another in space, and you would be surprised at the changes one might wreak on an unsuspecting world.Girls, Technology, The TARDIS, Magic and Music. Harry's starting a new life after his old one get's kinda boring.
When the Halloween spell ends Xander finds himself still in full possession of Ion's powers, adding to this the mystery that there might be someone else in Sunnydale who has kept their powers. someone that went as Supergirl, adding to this the usual school time hassles and troubles that come with being friends with a slayer.
Xander finds out that being nigh-omnipotent and having god like power's isn't all that great. especially when grief comes calling and he has so do everything he can, maybe sacrificing his own humanity in the process to save the ones he loves.

if you don't recognize the characters or want more information about them check the links below. Enjoy - Methos
ION - http://www.comicvine.com/kyle-rayner/40431/
Supergirl - http://www.comicvine.com/supergirl/2351/
Batgirl - http://www.comicvine.com/batgirl/1701/
Approached by the US Government, Alexander Harris finds himself with a job he'd never imagined in his worst nightmares. Train a team to take the fight to the supernatural, on or under the Sea, in the Air, or on Land.
Welcome to Seal Team 13.
The Only Easy Day, Was Yesterday

It was a Halloween like none other, and something had changed Xander, now he just had to find some way of learning about what had changed, and how to control his new power...
Harry Potter (Primary Universe and characters), Buffy (Secondary characters though Xander is the focus of the series), Babylon 5 (YAHF Costume universe)
Out Of Ruin, Emerges Hope, A New Hero rise’s
YAHF. Xander touches the heart of the dragon, and is changed forever. You didn’t think the Slayer was the only legacy power did you?
Spoilers for the Immortal Iron Fist as the series goes on.