I have tried to make this work for almost two years and I just can't come up with anything but crap. So I am putting up my plot bunny for adoption. I have two versions of it (think of them as twins). Here is one:

Spike is still incorporeal and is the History of Magic professor at Hogwarts
Xander is the DADA professor
They are in an established relationship
Spike can touch Xander and Xander can touch him only when Xander is wearing the amulet
They are there to help Harry win the war, and in exchange Dumbledore will help turn Spike corporeal.

and two:
same thing only Xander is de-aged and asked to pose as a student and befriend Harry in order to better help him prepare for the war. He also has to deal with trying to hide his ongoing relationship with Spike.

I really want to see both stories written. If you decide to take one on, please let me know. Happy writing. :)

PS. I would love to see more than one author's take on these stories, so don't think you can't throw your hat into the ring just because someone already did. :D