One great 'knd crossover fanfiction' could be one that crosses over with 'Teen Titans'. The combination of the two universes can bring about really interesting scenarios where the Kids Next Door team up with the Teen Titans to fight a common enemy. It can show how the different powers and personalities interact.
Another unique aspect is the character interactions. Sonic is a heroic figure in his own right, and the KND kids are also heroes in their own world. When they meet, there are clashes in their ways of doing things at first. But as the story progresses, they learn from each other. The KND might learn about Sonic's more straightforward approach to fighting evil, and Sonic could learn about the KND's clever use of teamwork and planning.
Another great one is 'Sonic and the KND: The Unlikely Alliance'. This fanfiction focuses on the challenges that Sonic and the KND face when they first form an alliance. It delves deep into the characters' personalities and how they have to adapt to work together. There are some really cool fight scenes and moments of character development throughout the story.
You need to study the characters of both 'knd' and the other series you're crossing with. Know their personalities, powers, and goals. If you're crossing with 'Avatar: The Last Airbender', understand how the KND kids would react to bending abilities and how the benders would view the KND's high - tech gadgets. Then start writing small scenes that involve the characters interacting.
One common theme is the clash of cultures. The Eds have their own goofy, self - centered world in 'Ed Edd n Eddy', while the KND are all about fighting against adults and saving the world. So when they cross over, there's often a lot of misunderstandings and chaos.
Well, one great 'Ed Edd n Eddy' and 'KND' crossover fanfiction is 'The Eds' Adventure in the KND World'. It has a really interesting plot where the Eds get mixed up in the KND's crazy missions. Another one could be 'Eddy's Plan in KND Territory' which shows Eddy trying to come up with his usual schemes but in the KND setting.
Adventure is another theme. They could be on a quest to find a lost KND weapon or to stop a new enemy. In these stories, Nigel's intelligence and Rachel's courage work together to overcome obstacles. Many fanfictions also explore their growth as characters within the KND universe, how they become better agents and better friends through their experiences.
I'm not sure who the absolute best is, but there are probably some talented writers on fanfiction.net. You can search there and look at the reviews to find authors with really good knd wally and kuki stories.
There might be a KND OC fanfiction where the OC is from a different time period but gets involved with the KND. Maybe an OC from the future who has advanced technology and shares it with the KND to fight off enemies more effectively. This could lead to some really cool adventures and interactions between the characters.
There could be a fanfiction where Wally and Kuki get lost in a strange place. In this story, they start to understand each other better as they try to find their way back. Kuki's optimism might balance out Wally's more laid - back attitude. They could encounter some friendly creatures along the way that help them in unexpected ways.