King would be a really fun character to include. His small size but big ego can create humorous situations in the human world. He could get into all sorts of mischief, like trying to convince humans that he's a powerful demon lord while also being fascinated by human things like toys and junk food.
Amity could also be a great addition. Her growth from a somewhat cold character to a more empathetic one can be further explored in the human world. She might struggle with fitting in at first but then find her place among human witches or those interested in magic.
Luz is an obvious choice. She can bring her knowledge of the Boiling Isles to the human world and show other humans the wonders of magic. Her enthusiasm and kind heart would make for great interactions with other human characters.
One idea could be a human character accidentally stumbling upon a hidden portal to the Boiling Isles and having to adapt quickly to the strange new world. They could form an unlikely friendship with a witch or demon and learn magic along the way.
Obviously, Spyro himself is a must - include character. He's the main character of the Spyro universe. Elora could also be included. She has her own magic and can interact with humans in interesting ways. Then there's Hunter, who could provide some action - packed scenes when paired with a human character in the fanfiction.
The friendly neighbor who lives on the outskirts of the woods can also be a great addition. They could bring news from the outside world, or be involved in a budding romance with one of the characters from the little house.
King might also be a character in time travel fanfiction. His search for his true identity could be further explored through time travel. He could go back to the time when his species was more prevalent in the Boiling Isles to learn about his origins, which would be an exciting adventure in the context of time travel fanfiction.
Luz is surely one of the main characters. She is often at the center of the action.
A scientist would be a great character. They could study the unique flora and fauna of the Jak and Daxter world and try to find ways to use their knowledge to help the inhabitants. For example, they might discover a new way to harness eco energy.
There might be some recurring human characters who are like outsiders coming into Elmore. For example, a human scientist who is there to study the strange phenomena in the town. Their inquisitive nature makes them interesting and popular among readers.
Lann and Reynn are obvious choices. They are the main characters in World of Final Fantasy and can serve as the link between different crossover worlds. Their ability to capture and use mirages makes them versatile in different settings.
Darwin is also quite popular. His relationship with Gumball is a big part of what makes the show great, and in human fanfiction, fans love to see how their bond might be different or the same in a human context. He might be the loyal best friend with a whole new set of adventures.
Another possible main character could be a rival collector. They compete with the main collector for the rarest and most valuable items from the Owl House. This rivalry could create a lot of tension and interesting plot twists, as they both try to outwit each other to get their hands on the most coveted collectibles.