... what are some of the things you would have done differently if you were the author of the Harry Potter series?

Well, acknowledging first, in gratitude to the author's fine effort, that hindsight is 20/20:

1. Put the Unbreakable Vow scene (at Spinner's End) ahead of Dumbledore's Ring Fiasco. In short: Snape agrees to murder Dumbledore, if necessary, before the subject of assisted suicide arises. Dumbledore's ring blunder would happen, later, then taking the moral onus off Snape (imo) but leaving the question of Snape's allegiance (allowing this wasn't satisfied Bk 4 when Snape showed in Barty's Foeglass). As written, Snape risked comparatively little; flipping the sequence, he risks, among other things, death via the Vow. The too: this order foreshadows his end.

2. Written an evil Slytherin who was attractive (charming, convincingly a friend to Harry, the Marauders, the Order....someone we admire) and written a Slytherin, other than Snape (presumably contemporaneous with the Trio), who actually was on the "good" side, attractive or not.

As is: there are no attractive Slytherins. None. Lucius corrupts the corruptible; Draco is a spineless prig; Slughorn is an indolent queen; Bellatrix is a psycho; Voldemort is a fraud and the rest are (mainly) brain-damaged thugs. We're told that Salazar and Godric were friends, and we're told the Founders speak thru the Hat in assigning Houses. Therefore: it cannot be there are no Slytherins attractive to any Gryffs, nor vice versa.

3. Revealed why it was Dumbledore let Sirius rot 12 years in Azkaban. It cannot be that Albus was entirely sure Sirius was Secret-keeper; we have seen Albus is full of self-doubt. It cannot be that Albus was so lazy or stupid so as never to check, via Pensieve, Prior Incantatam (sp?)or elsewise.

4. Re-written 7 Potters so that Harry was none of the 7 decoys. That was idiotic.