librasmile has created a rather intricate world around Severus Snape, with lies and adultery and Death Eaters, all culminating in a Hogwarts secretary trying to snare him (in the sexual sense). All of the stuff I've read is fully capable of being launched out of the Harry-Potterverse, but
Confessions of a Cornwall Grad is the longest and most intricate of her works. (Cornwall is the place that you go if you aren't "magic enough for Hogwarts.") Unlike some of her other stories, you barely have to have read any canon to appreciate it, and I think it's a shame so few people have checked it out.