DB Heroes doesn't follow the manga exactly. It takes inspiration and builds on it, making changes to suit the specific requirements of the series. However, the core elements are still recognizable.
Vegeta is also very likely to be a hero in the new story. He has grown a lot throughout the Dragon Ball series and in DB Legends, he might play a crucial role in fighting new threats. His Saiyan pride and his determination to surpass Goku could drive the story forward in interesting ways.
One common theme is the clash of different powers. Naruto has his ninja abilities like chakra and jutsu, while Janemba has his own demonic powers. Fanfictions often explore how these powers interact when they meet. For example, if Naruto tries to use his Rasengan against Janemba's energy blasts.
The length of DB mangas differs. It depends on factors like the story's complexity and the author's vision. Some might run for just a few volumes, while others could have dozens.
One interesting crossover could be a scenario where Harry Potter and his friends encounter Janemba's chaotic powers in the wizarding world. Maybe Janemba's otherworldly abilities disrupt the magic there, and Harry has to use his knowledge of both magic and the strange new power he's facing to set things right. For example, he might combine spells with his natural courage to combat Janemba's minions.
One popular 'Naruto Janemba fanfiction' could be 'Naruto and Janemba's Unlikely Alliance'. In this story, Naruto might find himself in a strange dimension where he encounters Janemba. The plot could revolve around Naruto trying to understand Janemba's powers and finding a way to either defeat him or make him an ally. It's an exciting read as it combines the unique elements of the Naruto universe with the otherworldly nature of Janemba.