There's also a possibility that it was an accident. Maybe Harry was exploring some ancient and dangerous magic, and something went wrong. A spell backfired and instead of achieving what he intended, it put a strain on his heart and led to the heart attack.
Maybe it's a result of a dangerous magical experiment gone wrong. Harry might be trying to test a new spell or a new form of magic that he read about in some old, mysterious book. But he doesn't fully understand it, and it backfires on him, leaving him badly injured.
It could be Dumbledore. If he is too focused on his grand plans against Voldemort, he might overlook Harry's well - being and needs, which could lead to Harry's death from neglect.
It could be a rogue Death Eater who has been hiding and waiting for the right moment to strike at Harry. They might use some forbidden dark magic to seriously harm him.
It could be Malfoy. He often has a vendetta against Harry and might have found a way to make him collapse during potions class, either by tampering with his ingredients or distracting him in a way that leads to a mistake in the potion - making process.
It could be Death Eaters. They might have finally found a way to capture Harry without alerting Dumbledore. His 16th birthday could be the perfect opportunity as everyone might be a bit more relaxed thinking he's not quite of age yet for the full - on final battle.
It could be a rogue Death Eater who managed to catch Harry off - guard. Maybe they were lying in wait and launched a surprise attack when Harry was alone, away from the safety of Hogwarts.
An unexpected character could be responsible. Maybe a character that Harry thought was harmless but was secretly envious of him. This character could have used some form of emotional manipulation, constantly undermining Harry's confidence and self - worth until his heart was broken and he died.
Well, it could be really tragic. Maybe his friends would be in shock and try desperately to save him. Hermione would probably use all her knowledge of magic to find a cure, while Ron would be by his side, feeling helpless yet determined to support his best friend.
It could be a case of betrayal from within. Maybe someone who was thought to be an ally, like a wizard who was secretly working for the dark side all along. They could have used a powerful and unknown curse to kill Harry in the middle of the night without anyone suspecting them at first.
Finding a 'Harry Potter heart transplant' fanfiction might be a bit of a challenge. Since the original series doesn't touch on such a medical - magical concept in this way. But if you do find one, it might offer a really interesting take on the character. For example, how would the magical world deal with such a Muggle - like medical procedure? Would there be spells to help with the recovery? Or would it be a sign of how much Harry has to rely on the Muggle world too? It could open up a whole new set of character developments and plot twists in the fan - created story.