HG/SS Rec - Second Life by Lariope
What might have been - a retelling of HBP & DH and what might have been if Dumbledore had married HG to SS so that he would have an ally. I enjoy this type of story - you have really got to know your canon to make it work. Another good example is Maya's "If You've a Ready Mind" which examines what might have happened if Draco had been sorted into Ravenclaw from Book 1 to 6.