"Your exploits at the Quidditch World Cup were fun, I daresay. . . but might not your energies have been better directed toward finding and aiding your master?"
—Voldemort talking to Lucius Malfoy in GoF

Point of Divergence: Voldemort's plans get slightly derailed when his Death Eaters get a little over-excited at the Quidditch World Cup.

In GoF, the rioters get a little bit more excitable than usual and decide to rampage a little. Ignoring Lucius Malfoy's advice of only torturing Muggles, Mulciber and Macnair decide to curse some Veelas. Bloodlust is readily cultivated in the din of chaos. Curses are exchanged, and now "dirty" half-breeds are indiscriminately murdered. The xenophobia reaches its peek when a stray Killing Curse hits a member of the Irish Quidditch team. Krum, being who he is, arms the Bulgarian Quidditch players and retaliate in self-defence.

Meanwhile, in the outskirts of the lynch mob, Barty Crouch Jr had gone off the bend after years of subjugation under his father. Year of Imperius Curses does that to your sanity. Instead of firing Morsmodre into the air, Crouch Jr chooses to use that split second to fire a Killing Curse at a trio of teenagers, realising that one of them was probably a Muggleborn. The Curse strikes a bushy haired girl, who falls to the floor as the life in her eyes departs her.

Burning with righteous anger, the girl's companions react with Stunners. Winky interferes, throwing the Invisibility Cloak back onto Barty Crouch Jr, when the Ministry officials stun all the people in the forest clearing. The girl's friends, as one can guess, are Harry Potter and Ron Weasley. Noticing their grief-laden expressions, the Ministry doesn't accuse them of the violence, unlike canon, and the two returned to Mr Weasley's custody.

Meanwhile, the casualties in this universe are slightly different. Various Veelas, who are affiliated with the Bulgarian Quidditch League, and members of the Irish National Team (Fan-favourite Barry Ryan and Seeker Aidan Lynch) have been either injured or tortured during the fray. A French national, Gabrielle Delacour, was also killed accidentally in the midst of the Curses. Unlike canon, the international community demands remuneration, and this incident has been elevated to an international crisis. An attack at an international wizarding game? Internationals have been killed?

When the French Ministry, led to Monsieur Delacour, decries the Fudge Administration for its incompetency in ensuring security at the premises, the Ministry of Magic is thrown in havoc. An emergency session of the International Confederation of Wizards has been declared by the Irish Ministry of Magic in Dublin, and Dumbledore suddenly has his attentions drawn away from the homeland. Anti-Fudge sentiment is rising overseas. The Tri-Wizard Tournament, as collateral, has been cancelled and boycotted by the Beauxbatons, derailing Voldemort's plans.

As both Harry and Voldemort deal with the aftermath of this escalating crisis, all parties are drawn to the 75th Session of the ICW in Dublin. The French delegation, led by Delacour, the British delegation from British Mugwump Dumbledore, and the Bulgarian delegation are all expected to confluence in Ireland in 1994. What will happen? How will Voldemort change his plans appropriately to factor in these changes?

How will a grieving, vengeful Harry factor into this new, international scenario?

 

You've done well. 5/5, would read.

 

HBP - after Harry injured Draco with Snape's trademark curse, his Dark Mark is revealed. He is immediately snatched by the trigger-happy Aurors, despite Dumbledore's and Snape's attempts to cover it up. Scrimgeour used the results of Draco's Veritaserum questioning to start a manhunt on all the DE's Draco knew. Snape is forced into hiding because Dumbledore's 'I trust Snape completely' doesn't work anymore. Harry's use of dark cutiing curse is overlooked because Draco tries to use Cruciatus on him, and BWL gleefully imagines using the same spell on Lucius and Bellatrix....

 

Now if we could only cross over Basilisk-riding Tom Riddle and T-rex-riding Harry Dresden, then we'd have the awesomest thing ever written since Harry fought astronauts with a groinsaw.

 

I remember discussing this with someone once.

Point of Divergence: anytime after Harry learns about legilimency, though I'm leaning to the summer after Book 5

What if Harry got a magical tattoo, possibly to protect against legilimancy, that started affecting his judgement?

I don't remember anyone with body art mentioned in the books. In this story, the reason is that magical tattoos tend distort one's magic over time. Harry is raised muggle, and is kept isolated from the traditional magical families, so no one ever mentioned that tattoos tend to be a bad idea. It never occurs to Harry that anything that affects one's brain is probably a bad idea anyway.

Harry goes through the Black library looking for information on occlumency, and finds a small, old book about mental defenses. Along with techniques for meditation and "clearing the mind," there is a sketch of a symbol or rune set that will provide a certain level of protection. Harry doesn't stop to consider that anything found in a library that leans heavily to Dark magic might not be a good long-term solution. The book suggests that the symbol be carved or painted on something that is then worn as jewelry. Harry is well aware that anything he owns might be lost, stolen by a fan or an enemy, or simply confiscated by Snape-being-a-jerk. All those have happened before, after all. Harry decides to get a tattoo.

When he mentions the idea to Ron, Molly Weasley overhears, and she throws one of her screeching fits. She doesn't explain that tattoos are a bad idea, she simply yells that none of her children are going to have something so disgusting. Harry never gets a chance to describe his idea, and Molly leaves him with the impression that she thinks tattoos are crass. Harry, being Harry, decides that what she doesn't know won't hurt him. That means that Ron can't be told either - he'll blurt it out in moments.

Harry is also practicing the techniques described in the book - mostly meditation and visualization techniques.

There is a strong lack of privacy at Hogwarts. The showers and the Quidditch locker rooms, in particular, are communal, and five boys in a dorm means that nothing stays covered for long. To avoid any more drama in his life, Harry decides to get the tattoo somewhere hidden; either the sole of his foot (using a potion to heal it immediately,) or possibly by shaving a patch of his scalp. Remember that his hair grew back overnight once when Harry was little.

After Umbridge's abuse and Dumbledore's abandonment the year before, and Molly Weasley's screeching, Harry is in no mood to confide in an adult.

He goes to a muggle tattoo parlor (there aren't any magical ones,) and he gets the tattoo artist to copy the symbol. You could have him have to use some sort of compulsion spell or potion to get the artist to use a special ink (actually a potion that Harry made.)

He never gets a chance to talk it over with Hermoine. She and Ron are busy playing with each others' ... hormones, and are completely refusing to consider his suspicions about Malfoy. By the time they actually get time to talk, Harry is already in hot water about his Potions work.

When Dumbledore notices that Harry is less vulnerable to legilimancy that he used to be, Harry gives him an evil look and snaps something about how once he found a book on occlumency techniques, it got much easier than trying to obey Snape's shouted orders. AD would drop the issue.

The symbol was never intended to be worn continually. The creator's idea was that the symbol would be on a piece of jewelry that would only be used when the owner was in public or suspected that he might be subject to a legilimancy attack.
The tattoo's malign effects take a while to show up. The crucio/sectumsempra incident in Myrtle's bathroom, as well as CAPSLOCK!Harry, could have been warning signs of the the tattoo affecting Harry. Harry was having trouble sleeping and was touchy anyway. Even his fairly obsessive interest in Malfoy could be a sign of a slightly unbalanced mind. This means that Ginny (who I do suspect of using love potions) would get a weird effect. AD is murdered, and Harry's suspicions are vindicated. Ron and Hermione are ashamed that they didn't trust him, and decide that any weirdness was their fault for their lack of support. This means that more warning signs are ignored.

I can see lots of ways for the tattoo to affect him - schizophrenia, fugue states, even worse sleep patterns... actually, the effects could be covered by an increased ability to concentrate. Fanon usually has occlumency having a positive effect on sorting memories and organizing new information.