Xander becomes Immortal to save the world/his girls/the neighbor's dog (reason doesn't matter), but fails to understand the full implications. He outlives them all, and their children and grandchildren etc. He sinks into depression eventually, but can't manage to bring an end to it all (hello, Immortal). His imperative from the PTB hasn't changed - he still fights the good fight - but he does it with such apathy that he's actively looking to be harmed. Enter Spike, who takes him by the ear and twists until Xander cries uncle. Spike has to teach him how to live again, and how to live well even without his family.
Anyone interested in taking this off my hands? The more the merrier!