Harry Potter and the Veil of Mystery

Harry Potter and the Veil of Mystery is the first in the "Post-OotP" trilogy by the author semprini. The next two stories are Harry Potter and the Ring of Reduction and Phoenix Intuition. You can find the stories at semprini's Blogspot page. Note that the fics Harry Potter and the Antiquity Link and Harry Potter and the Amulet of the Moon are not part of the series and constitute a separate post-Deathly Hallows arc.

There are some spoilers for Harry Potter canon through to Order of the Phoenix.

Tropes used in Harry Potter and the Veil of Mystery include:

Big Bad: Voldemort. In spades. See below.
Cynical Mentor: Severus Snape.
Eccentric Mentor: Albus Dumbledore.
Cool Teacher: Harry Potter. He becomes the Defence Against the Dark Arts teacher in Veil of Mystery and Ring of Reduction. Albus Dumbledore, as well.
Stock Aesops: Many of the older characters who speak to Harry often to do to illustrate moral or ethical messages. As an example, Hugo Brantell, a quasi-psychic news reporter, explains the "facts of life in the media" in order to explain to Harry why he needs to keep engaging with the public, even though he has a very strong distaste for the media generally due to the abominable way the Ministry and the Daily Prophet treated him during his fifth year at school.
Aesoptinum: Harry Potter's ability to use the magic of love to generate shields against the Unforgivable Curses is rooted in a mindset that requires a state of mind akin to that attributed to Jesus Christ's "they know not what they do" level of forgiveness.
Moral Dilemma: Many characters in this fic arc discover truths about themselves, some pleasant and some very unpleasant, through engagement in moral dilemmas, or in some cases, the blowback from decisions they made in canon or in the fic arc. Example: Hermione Granger's decision to forbid Rita Skeeter from writing any articles of any kind eventually causes a very nasty situation for her and her friends.
Deal with the Devil: Voldemort purposely mutilates the minds and souls of his Death Eaters in an act called the Cleansing, which removes the ability to feel happiness except through violent acts. Snape has suffered this and wishes to have it reversed, but Dumbledore was unable to effect this. Harry Potter eventually figures out how to reverse the process. He crows about how he even did this to himself and gleefully gloats that he believes this will help him win against Harry. Draco Malfoy is the counterpoint to Snape's wish for redemption as he happily accepts the Cleansing and refuses to have it reversed even when Harry offers to do this for him. He's not a nice person in this arc.
Anti-Hero: Severus Snape. After killing his mother, he had a My God, What Have I Done? moment. Snape was sufficiently regretful to go to Dumbledore and try to redeem himself by means of spying for him. Afterwards, Dumbledore was able to treat the symptoms of the Cleansing, but not cure the underlying cause. Finding the ultimate cure fell to Harry Potter. Additionally, Snape's point of view, in contrast to some of the more nuanced moral perspectives offered by others, is extremely pragmatic based solely on the goal of defeating Voldemort. It is this which prompts him to assassinate Rita Skeeter as soon as he realizes what she has found out.
Grey and Gray Morality: There's moral ambiguity to go with the above tropes. Harry's initially relatively black and white moral system takes on a leavening of grey as he tries to reconcile the real-world acts he sees with the moral code he's developed, which was in response to regarding the abyss of casting the Cruciatus Curse on Bellatrix Lestrange in Order of the Phoenix. Neville also ends up discovering the problems with He Who Fights Monsters when he encounters Lestrange.
Heroic Sacrifice: A prominent example is Dumbledore purposely throwing himself through the Veil in the Ministry during a battle with Voldemort, assisting Harry in his task to defeat Voldemort once and for all.
Shipping: Fairly bog-standard, but there's a few surprises. Harry Potter dates Ginny Weasley, Ron Weasley dates Pansy Parkinson, and Hermione Granger dates Neville Longbottom. It's mostly kept in the background, but the romantic trials and tribulations of the teenagers do occasionally provide fodder for the above Moral Dilemma issues.
Heel Face Turn: Pansy Parkinson spies for Harry in Slytherin House and provides a crucial warning to Harry, saving his life when a student tries to use a machine-gun to kill Harry in the Great Hall.
Chekhov's Gun: Actually, it's more like Chekhov's Small Arms Factory in these fics. Everything the author introduces as a special skill or special item, even if not obvious, gets used in the fic arc to accomplish something. Example: Some artifacts being smuggled in by Draco is briefly discussed at one point in the fic arc. Some chapters later, Draco Malfoy uses one of them to kidnap Ginny Weasley.
The Woobie: Blaise Zabini. He ends up basically being cannon fodder. He's gay, and Snape, in particular, has a very bad reaction to finding this out. Blaise ends up being killed off in a Death Eater attack.
Ascend to a Higher Plane of Existence: Through a very unique set of circumstances, Dumbledore finds a way to "link" to Harry and introduce him into the afterlife in a limited way. This not only allows Harry to get critical information in a way that would otherwise be impossible, but facilitates the reintegration of Snape by reversing the Cleansing, as well as giving his friends closure. As an example, Neville's grandmother and his parents are able to give him final messages.
Right Makes Might: Mostly averted, but it exists in a small measure when Harry comes up with the "Imperius Charm" which allows him to manipulate people in very specific circumstances. His acts can be justified on the basis of some woo-woo involving the nature of the power behind the magic of love and that, by authorial intent, it paves the way to discovering how to reverse the Cleansing.