Maybe in the fanfic, the Resurrection Stone was never truly lost. It was secretly being protected by a group of centaurs in the Forbidden Forest. A young Hogwarts student stumbles upon this secret while exploring the forest. The centaurs test the student to see if they are worthy of handling the stone. If they pass, they are given the stone to use for a short time, but they must return it unharmed. This could lead to a series of adventures as the student tries to decide who to bring back, if anyone.
A plot could involve a dark wizard who steals the Resurrection Stone from a museum where it was being held for study. This dark wizard plans to use it to bring back all the dark wizards from the past to form an army. The Order of the Phoenix has to find a way to stop him. A young witch or wizard, who is an expert on ancient artifacts, joins the Order to track down the stone and figure out a way to destroy it or at least nullify its power.
One interesting plotline could be that a new character discovers the Resurrection Stone and uses it to bring back a minor character from the series, like Colin Creevey. This creates a whole new dynamic as the wizarding world has to deal with someone who was brought back from the dead. Another plotline might involve the Stone being used to communicate with the founders of Hogwarts, seeking their wisdom to solve a modern - day problem in the wizarding world.
Perhaps a fanfiction where the trio - Harry, Ron, and Hermione - encounter a different set of magical creatures on their way to stop Voldemort from getting the Stone. For example, they meet a friendly group of house - elves who help them with clues. This would add a new dimension to the story and show the different aspects of the wizarding world.
Well, you could have a fanfic where Voldemort uses DND - style dark magic to open portals to other dimensions. The Order of the Phoenix has to quickly learn DND - style combat and magic rules to fight off the new threats that come through these portals. Harry could be sent on a special mission to find a DND - style mage who can teach them the counter - spells. Meanwhile, the Death Eaters are also trying to master this new form of magic for their own evil purposes. This would create a really exciting and unique story.
Perhaps a plot where the protagonist can choose to either side with the Ministry of Magic during a time of political unrest within the wizarding world or join a rebel group. This would involve different magical battles and moral dilemmas depending on the choice. In the 'harry potter cyoa fanfic', this could show different aspects of the wizarding society that we don't often see in the original books.
A cool plot for a Harry Potter Twilight fanfic could be a competition. The wizards from Harry Potter and the vampires and werewolves from Twilight compete in a series of challenges. These challenges could test both magic and the unique abilities of the Twilight characters. Or perhaps a love triangle between a Harry Potter character, a Twilight vampire, and a Twilight werewolf. It would be interesting to see how the different cultures and powers interact in matters of the heart.
Another plot could focus on the lives of the secondary characters from the original series. For example, Neville could be the head of Herbology at Hogwarts and discovers a new, dangerous plant that has some sort of connection to an ancient magic. The 'carry on fanfic' could then be about how he and his students deal with it, with some cameos from the main characters.
One idea could be that a dragon escapes from Gringotts and Harry has to use his skills to track it down. Maybe the dragon has a special connection to a powerful dark magic item that Voldemort was after.
One idea could be that Voldemort somehow enters the Runescape world to seek more power. Harry and his friends follow him there to stop him. They have to learn the rules of Runescape quickly to be able to fight Voldemort. Another idea is that a Runescape wizard discovers a portal to the Harry Potter world and brings some of the unique items from Runescape, like special runes, which causes a stir in the wizarding world.
A great plot for a Harry Potter academy fanfic might be a time - travel scenario. A group of students accidentally travel back in time to the founding days of Hogwarts. They have to navigate the different rules and attitudes of that era while also trying not to change history too much. Or perhaps there's a plot where a student is wrongly accused of a major crime at the academy and has to prove their innocence. They'll need to use their wits and the help of their friends to find the real culprit.
Perhaps in the fanfic, the snake is a key to unlocking an ancient magic that has been lost in the wizarding world. This could lead to a whole new adventure for Harry and his friends as they try to understand and use this magic.