I think Voldemort would be the most interesting. We rarely get to see his perspective in the original series. In a fanfic where Harry talks to him, we might get to see a more complex side of Voldemort. Maybe he'll show some vulnerability or a different motivation than just pure evil. It could be really fascinating to see how he reacts to Harry's words and if he is capable of any change.
It could be both Harry and Voldemort equally. Harry because he is the hero trying to reach out to the ultimate villain. His character growth and the development of his ideas during the conversation would be interesting to follow. But Voldemort too, as his reactions to Harry's attempts at communication could be very unpredictable. He might show a side of himself that we never expected, and his internal struggle (if there is any) between his dark desires and Harry's words would make him a very engaging character.
I think Harry would be the most interesting. He has always been the hero who stands against Voldemort. But now as partners, he has to deal with his own internal conflict of teaming up with his arch - enemy. His struggle to balance his moral values with the need to work with Voldemort would be really engaging to read about.
One possible plot could be that Harry tries to reason with Voldemort about the futility of his dark ways. He might talk about how much destruction Voldemort has caused and how many lives have been lost. Harry could also try to appeal to any remaining shred of humanity in Voldemort, perhaps by bringing up his own orphan status and how they both lost their parents in a way, though for very different reasons.
I think it would be really interesting to focus on Hermione. Her thirst for knowledge would make her eager to learn about the world of Eragon, especially the dragon - riding and the different types of magic. She could also bring her logical thinking and problem - solving skills to the table, which would be very useful in both worlds.
It would probably be Harry Potter himself. He's from a world full of magic and mystery, and seeing him interact with the very real and regal world of the Queen would be fascinating.
Pairing Supergirl with Severus Snape could create a really dynamic relationship. Snape has his own dark past and complex personality. Supergirl could potentially see the good in him and try to bring it out. They could have a bit of a clash at first, with Snape being suspicious of her, but over time, they could develop an unlikely friendship or at least a mutual respect as they face common enemies in the wizarding world.
Definitely Dr. Strange. He already has a connection to magic, so in a Harry Potter x Avengers fanfic, he could bridge the gap between the two worlds. He could learn new spells from the wizards and share his knowledge of the mystic arts. His ability to manipulate time and space would be really useful in the wizarding world, and he could help the Avengers and the wizards deal with threats that involve both magic and science.
One interesting concept is when Voldemort cheats by infiltrating Harry's dreams. He can manipulate the dream world to make Harry doubt himself or give away secrets. It adds a psychological aspect to their conflict. Another is when he cheats by using ancient magic that no one expects. This often leads to Harry having to research and learn new magic quickly to counter it.
Wonder Woman and Ron Weasley could be an interesting combination. Ron is often underestimated but has a lot of heart and loyalty. Wonder Woman, with her strength and noble character, could inspire Ron to be more confident in himself. Ron could introduce Wonder Woman to the fun and sometimes chaotic side of the wizarding world. They could work together to fight off enemies, with Ron using his magical skills and Wonder Woman using her combat and leadership skills.
Hermione and Alice would also be a great pair. Both are highly intelligent and studious when it comes to magic. They could compare their different educational systems, with Hermione being schooled in the traditional Hogwarts curriculum and Alice having learned at Brakebills. They might start off as a bit competitive but then realize they can achieve more by working together, like researching a new and dangerous magical phenomenon that affects both their worlds.
Luna Lovegood and Jacob could be an unexpected but great combination. Luna's unique view of the world and her belief in the unseen could pair well with Jacob's connection to the spirit world as a werewolf. They could embark on a journey to discover a new kind of magic or power that exists between the two universes, and their interactions would be both humorous and heartwarming.