A common theme is power. Since Harry is a king, power dynamics play a big role. His relationship with his mate is often influenced by his position of power. Another theme is the merging of two different worlds, like the magical world of Harry Potter and the supernatural Twilight world.
One popular 'twilight harry is a king mate fanfiction' might be 'The King's Chosen' where Harry's character is reimagined as a powerful king - like figure in the Twilight universe. It often has elements of him finding his mate in a unique and unexpected way.
Obviously, the Twilight King is one of the main characters. He is likely to be a powerful and mysterious figure with his own set of powers and a kingdom to rule. His mate is another main character, though we don't know much about this character yet. There might also be some side characters like advisors to the king, or perhaps other magical beings in the kingdom who play important roles in the story.
Another plotline could involve the discovery of the mate. It could be a situation where the Twilight King doesn't initially realize that a certain character is his mate. As they interact more, there are signs and prophecies that gradually point towards their special connection. This could lead to a journey of self - discovery for both characters.
Harry is a must - be main character here. His mate, of course. But also, there could be entirely new characters created specifically for this fanfiction. For example, some wise old elves who have knowledge about the history of the king mate concept. Or perhaps some young, brave wizards and witches who are inspired by Harry and his mate and join their cause. All these characters would interact and shape the story in 'harry is a king mate fanfiction'.
There's also a fanfiction where the mates concept is explored deeply. It might be titled something like 'Finding the True Mate: Harry Potter and Twilight'. In this story, characters from both series are on a journey to find their true mates, and it weaves in the unique aspects of each world, like Hogwarts' magic and the vampire abilities in Twilight.
Well, in some 'harry is a king mate fanfiction', there might be a plot where Harry discovers his new regal status and has to learn the ropes of being a king while also dealing with his mate. For example, he could be facing political intrigue within the magical kingdom and his mate might be his source of support and also a key to solving some of the mysteries related to his new role.
Another possibility could be a new character created specifically for the fanfiction. This new character might have unique abilities or a personality that attracts Edward in ways Bella didn't. For example, she could be a vampire from a different coven with a very different view on life and love, which makes Edward question his feelings for Bella and consider this new character as a potential mate.
Common themes in 'Twilight Mate' fanfiction are loyalty within the mate relationship. The characters are usually extremely loyal to their mates, no matter what challenges they face. Another is the exploration of the unique abilities that come with the mate bond. Some stories show that mates can communicate telepathically or have enhanced strength when together. Also, the theme of fate and destiny regarding mates is common. It's often shown that the characters are fated to be together, and the story unfolds around this pre - determined connection.
First, you need to do a lot of research on both Harry Potter and Twilight universes. Know their characters, settings, and rules well. For example, understand how magic works in Harry Potter and the nature of vampires in Twilight.