One of the frustrating things about the HP books is that they establish these interesting if rather fragile categories and then go ahead and spend the next several thousand pages talking, in the main, about Gryffindors, who are necessarily varied enough to make one wonder why anyone takes sorting seriously at all, when you have Peter Pettigrew in under the umbrella of "courageous". But yeah, when the abilities to prioritize and solve problems without martyring yourself and run cost-benefit analysis on a situation are all repeatedly described as Slytherin traits, it gets a little hard to find plot-furthering, competent adult characters who wouldn't fit comfortably within that purview.

(It's been a while since I read any Harry Potter fic that wasn't a Homestuck crossover, but when I did I was pretty fond of Snape/Harry too. Though I think my tolerance for Snape himself has lowered a little in the intervening period-- more because of his Gryffindor qualities than his Slytherin ones, aha. All that damn everlasting unrequited love!)