There’s been a lot of chatter about “what if the Goblet of Fire had selected the Slytherin guy instead of Cedric Diggory?” and how it would have been super touching for a Slytherin to get killed by Lord Voldemort which would rally the Slytherins to join the others in opposing the Death Eaters… Slytherin has always been designated the “evil” house and any attempt to add some complexity/depth to it is always welcome. It would be preferable, however, if such an approach was built on something plausible. Most seem to be presuming a “Cedric Diggory but Slytherin” stand-in is chosen but that’s not correct. What would happen is that the Slytherin champion would be a complete asshole and his interactions would completely alter the dynamics of the novel, not necessarily for the better.
From what information is given in the books, the Slytherin guy who put his name in the Goblet of Fire is named Cassius Warrington. He is a recurring character in books 3-5 and is a total dickbag whose only character traits are that he’s big, looks like a sloth and that Dolores Umbridge thought highly enough to place him on the Inquisitorial Squad. Yes, Umbridge didn’t show up until the next year but we can look at who else was placed on that squad to get insight into his character. Other “inquisitors” include Filch, Draco Malfoy, Crabbe, Goyle and Pansy Parkinson (who openly spoke up about handing Harry over to Voldemort before the final battle in Deathly Hallows). If people are often judged by the company they keep, then Cassius Warrington is just another asshole in Slytherin.
The scenario also changes the whole relationship dynamic of two Hogwarts champions. Cedric Diggory and Harry Potter respected each other because they both valued fairness and not just in Goblet of Fire. When the Dementors cause Harry to lose the Hufflepuff/Gryffindor match in Prisoner of Azkaban, Cedric proposes replaying the match. Harry warns Cedric of the first task in Goblet and Cedric returns the favor later with the second task. If Cedric’s replaced with a Slytherin champ, all that goes out the window: Harry’s not going to warn some jerkass from the most hated house of the first task and a Dolores Umbridge-approved individual isn’t going to help Harry with the second task.
If that’s not enough of an argument, consider this: In Order of the Phoenix, Graham Montague, another Slytherin on the Inquisitorial Squad, is pushed into the Vanishing Cabinet by Fred and George Weasley. This is after Montague tries a dick-move power play. Montague only gets out because he Apparated but he almost dies in the process. He spends several weeks in the hospital, long enough for his parents to come visit. When Hermione proposes telling Madam Pomfrey what happened to Montague, Ron and Harry shoot her suggestion down.
In other words, the hatred between Gryffindor and Slytherin is so strong that our heroic protagonists were willing to let a Slytherin suffer from near-fatal injuries than do anything to expedite his treatment! The Slytherin/Gryffindor hate even continues when Lord Voldemort’s Death Eaters are outside Hogwarts near the end of the seventh book. Voldemort asks only for Harry to be turned over to him. Pansy Parkinson, a Slytherin, is the only person who suggests taking the deal. Slytherin walks in step with Voldemort to the extent their entire house leaves with Filch and the under-aged, rather than side with Harry and fight Voldemort.
Now, the only reason Cedric ended up in the graveyard at the end of Goblet is because he and Harry made a conscious decision to touch the Triwizard Cup at the same time. It’s already been established that a Gryffindor and a Slytherin aren’t going to have that kind of camaraderie, so the only way Cassius ends up in the graveyard is if Barty Crouch Jr. (who’s manipulating the events of the maze) allows it. The only reason he would allow that is if Cassius is complicit with the Voldemort resurrection plot or because of writer fiat.
Even if we allow for Cassius to end up in the graveyard for whatever bullshit reason, what’s to say Voldemort kills him like Cedric? By all accounts, Cedric was a pretty cool guy and not the type to take suggestions from an evil overlord. Voldemort had no use for him because he couldn’t see a use for him. A Slytherin, on the other hand, would be more susceptible to suggestions. Perhaps, as described above, Cassius is keen on Voldemort’s resurrection. In that case, he’s just another Death Eater at the graveyard. Maybe his survival instinct kicks in and he runs for the portkey, leaving Harry in the graveyard with no way out.
Finally, if we play along with the idea that Cassius gets murdered by Voldemort, there’s a few problems that crop up. (1) There’s no guarentee that Harry brings Cassius’ body back, if Cassius even asks such a thing. (2) The memorial feast for Cedric would be more like a memorial roast for Cassius, since the student body would remember him as a big Slytherin dickhead. Even the Slytherins would most likely discuss whether Cassius was “evil enough” or that he was “mudblood sympathizer” and that justified Lord Voldemort murdering him. (3) The Ministry of Magic did a really good job covering up Cedric’s death and ostracizing Harry and Dumbledore as conspiracy theorists. Substitute Cassius in for Cedric, add in Death Eater parents covering up the truth and there’s not (m)any Slytherin students who are going to be joining Dumbledore’s Army. (4) Any Slytherin student that did join up would be doing so out of self-preservation, not because they realize Lord Voldemort is a threat to the entire wizarding world (the conclusion the other houses correctly reach).
People have been wanting more complexity out of Slytherin since they’ve been shoe-horned (slug-horned?) into the “obviously evil” Hogwarts house. This fan theory is an attempt to address that but has no basis within the rules of Harry Potter. Much like everyone else who was sorted into the house, the Slytherin contestant is a total jerkwad. Because characters like Cedric Diggory either don’t exist or speak up in Slytherin, we can’t just sub in a “Cedric Diggory but Slytherin” replacement without changing the plot. What would happen is that Cedric Diggory would be replaced by a complete asshole who’s another obstacle Harry has to overcome during the Triwizard Tournament. Much of the narrative would have to be tortured to get a Slytherin champion to the graveyard and there’s no guarantee Voldemort would kill him. Even if he did die, there’s no reason to think his death would galvanize Slytherin to take up arms against Voldemort considering Cedric’s couldn’t do that for the wizarding world after the Ministry of Magic suppressed the truth.