One idea could be Harry Potter arriving in Skyrim and using his magic to combat the dragons. Maybe he teams up with the local mages and they learn from each other's magical techniques.
A possible plot could involve a Dragonborn who discovers a hidden portal to the wizarding world. In the wizarding world, they learn about magic wands and spells. The Dragonborn could then combine their Thu'um powers with magic spells to fight against dark forces that are threatening both worlds. For example, Voldemort could team up with Alduin in some way, and the Dragonborn and Harry Potter have to work together to stop them.
Maybe Harry gets involved in the civil war in Skyrim. He could use his Hogwarts-taught dueling skills to fight in battles, and his knowledge of magic could be used to solve some of the political intrigues. He might also try to find a way to bring peace to Skyrim using his unique perspective on magic.
One idea could be that Hogwarts is magically transported to Skyrim. The students have to deal with the cold, the dragons, and the different magic systems. Another could be that Harry and his friends are sent on a quest in Skyrim to find a powerful magical artifact that could help defeat Voldemort. Maybe they have to team up with the Nords and learn their ways of fighting and magic.
One interesting concept could be the combination of magic systems. In Harry Potter, there's wand - based magic, while Skyrim has shouts and spellcasting in a different way. A fanfic might explore how wizards from Hogwarts would adapt to the magic in Skyrim, or vice versa. For example, a Hogwarts student trying to master a dragon shout.
The SI could be a warrior who was raised by dragons. They have dragon - like combat skills and a code of honor similar to that of dragons. In the story, they might be searching for a way to stop an ancient evil that threatens both dragons and humans. They would travel across Skyrim, facing various foes and making allies among different races, all while trying to keep their dragon - influenced identity a secret.
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.