It's probably been mentioned before, but it's definitely worth bringing up again even so - Disobediencewriter's The House of Harry Potter Rebuilt has some great magical theory, and also heads boldly away from canon events to tread refreshingly uncommon ground. 98k words, last chapter was born as the sixth month died but as only the fifth in over a year that's still not out of the bounds of a successful resurrection. (Especially if the author suddenly gets a bunch of comments... 
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