There was something really appealing to the Potter Universe for me... i enjoyed it when i was a kid, both the books and the movies, although they seemed a bit rushed after the 5th movie. Not to mention writing pages of RPG on a certain forum i really enjoyed & co, i also played Pottermore, but well, this big interest about the details wasn't only about HP and all... now the whole concept saddens me a little, i don't know why, but there's still something special aout it.
She mentioned in a few places about being interested and studying astrology before considering developping her characters' personalities. The books are full of such symbolism: the Centaur Firenze who introduced Harry to the idea of foretelling the future, the name of important characters inspired by the big fixed stars (ex: Sirius Black able to transform into a Dog; Bellatrix Lestrange; Pollux Black, Arcturus Black, Regulus Black; Andromeda Tonks; Cygnus Black & co), the 4 Hogwarts Houses and the Four elements, Witchcraft and the "stick, circle and triangle" symbolism and basically the books in themselves, with a lot of 8th/12th house matters, by definition. They still present a Piscean dreaminess and confusion sense i'd say, but it still doesn't get "blacker" than some books about war and human tragedies in my opinion (and i see the 12th House as less "black" or "intense" than the 8th one so Rowling having planets in the 12th might be as well a good supposition i'd say), well, these books still are mainly destined for the young ages... the plans of existence aren't as definitively shaped or heavy like those of Tolkien, although i still enjoyed a lot his books and Silmarilion too. It was something different, but able to wake up feelings inside the reader, more profound than other books because they touch subject that aren't usually easily touched.
On another note, now that i try it, i think it's a bit weird to see or analyse a book only from an astrological perspective, or even to concentrate mainly on it in life in general. There are so many other "planes" of existence to "investigate" and live in, at the same time even, that choosing only one instead of combining and working with them is equal to limiting yourself in a way (unless it's for a short period of time or has an important reason behind).
Well, going back to the topic, I'd vote for Virgo as her ASC in the end, or something Watery
Her ascension was dictated by a strike of luck (Jupiter conjucting something in her chart like the MC/NN as otherwise there are many squares), a lot of hardships behind (the oppositions to a retrograde saturn on an interesting degree and squares to Jupiter), persistance (unafflicted Mars able to operate on subtler levels as well) and imagination (well aspected Neptune in a more money oriented House or Saturn in the 2nd as someone stated?).
Mercury on Regulus in an interesting one there.
But what she also lacked in her life, as sources say, was a combination of emotional and financial stability of home. The chart on astrotheme shows Saturn in the 4th, and this might be a very sugestive aspect. Her Leo Sun doesn't seem to stand out too much, and seeing her chart, there aren't many aspects to it. The exterior is challenged, while internalisation gives more content feelings and, like in every Leo, there's an always present sense of happiness or desire or something good even thought he person seems "dry". Oh, well...
But she jokes in interviews about not being aware of her surroundings pretty often. More importantly I like the other placements a Scorpio or Virgo Rising give.