Obviously, Willy Wonka himself is a must - include. He's the heart and soul of the factory. Charlie Bucket is also great as he represents the pure - hearted visitor. The Oompa - Loompas add a lot of fun and mystery with their strange songs and unique appearance. Veruca Salt can bring in some drama as she was the spoiled brat in the original story.
I think Willy Wonka and Charlie Bucket are the top choices. But don't forget about Mike Teavee. He can add a technological aspect to the fanfic if you want to explore modern - day elements in the world of Wonka. And the Oompa - Loompas can be developed further, like giving them individual personalities rather than just being a group that sings moral - teaching songs.
In a crossover with 'Peter Pan', Peter himself would be a great addition. He could fly around the chocolate factory, causing mischief among the Oompa - Loompas. Wendy could also be there, and she might befriend Charlie and help him understand some of the more complex parts of the factory. Tinker Bell could add some magic sparkles to the already magical environment.
Dumbledore could also be a popular character to include. His wisdom and power would make for an interesting interaction with Willy Wonka. He could be intrigued by Wonka's magic, which is different from the wizarding magic in the Harry Potter series. And the Weasley twins might be included as they could get up to all sorts of mischief with Wonka's magical candies.
The Oompa - Loompas are a must - include. They can add humor and a touch of mystery to the story. For example, they could be the ones to guide the reader through the factory when Willy Wonka is busy.
Sonic himself is a must - include. His speed and adventurous spirit drive the story forward. Tails is also great as he can provide technical support and is Sonic's loyal friend. And Dr. Eggman, as the main antagonist, always adds conflict and excitement to the story.
Vegeta is a great choice. His rivalry with Goku adds a lot of drama and action to a fanfic. Also, Piccolo, as he has a long history with Goku and has been both an enemy and a friend. Another good character could be Gohan, Goku's son, as it can explore their father - son relationship further in the fanfic.
Another good character to include is Lex Luthor. He could be the mastermind behind a plot to drive a wedge between Batman and Wonder Woman. His devious mind and resources would make him a formidable antagonist, and his interactions with the two heroes would add a lot of drama to the story.
Optimus Prime is a great choice. His leadership and wisdom can add depth to the story, and his relationship with Bumblebee can be explored further. Also, Sam Witwicky, as he has a strong bond with Bumblebee in the movies. Their friendship can be a central part of the fanfic.
Definitely Naruto himself, as he is the main character of the original series. Sasuke is also a great choice as his complex relationship with Naruto can add a lot of drama. Kakashi is another good character to include. His wisdom and unique abilities can be very useful in the story. And Sakura, her growth as a ninja can be explored further in a fanfic.
Well, obviously Armikasa themselves are the main focus. But you could also include their friends. For example, their comrades who are with them in training or on missions. These characters can add more depth to the story by providing different perspectives and interactions.
Nami would be a great character to include as she has a deep - rooted history with Arlong. Their interactions can add a lot of drama and emotion to the story. Luffy is also a good choice as he is the main protagonist and his values of freedom and justice would contrast sharply with Arlong's tyranny.