One plot idea could be that the Dragon Balls are accidentally brought into the Evangelion universe. This causes a stir as different factions in the Evangelion world, like NERV and SEELE, try to get their hands on them for their own purposes. Meanwhile, the Z - Fighters follow the Dragon Balls and end up in this strange new world. Another idea is that an Eva unit gains the ability to use ki, like a Saiyan. This Eva then goes rogue and starts causing chaos, and it's up to both the Evangelion pilots and the Z - Fighters to stop it.
A possible plot idea is that Godzilla is drawn to the Geo - Front in Evangelion. The reason could be the powerful energy sources there. The NERV organization is then forced to either work with Godzilla or find a way to get rid of it. They might study Godzilla's biology to see if they can use any of its abilities to enhance the Evas. Another plot could be that an Angel takes on the form of Godzilla to deceive the Eva pilots. This would make the pilots question their usual tactics when facing an 'Angel' as Godzilla has different strengths and weaknesses compared to the normal Angels. Also, a plot where Godzilla's presence causes a shift in the power dynamics between the different factions in Evangelion, like SEELE and NERV, could be really interesting.
One idea could be an Evangelion and Gundam crossover. Picture the Eva units in a battle alongside Gundam mechas against a common enemy. Another could be a crossover with Neon Genesis Evangelion and Sailor Moon. The pilots of the Evas could meet the Sailor Scouts and combine their powers to fight a new threat. And a third idea might be an Evangelion and Pokémon crossover, where the Angels are somehow related to powerful Pokémon and the Eva pilots have to use Pokémon in addition to their Evas to save the day.
A great plot for a Naruto DBZ crossover fanfiction might be that there is a rift in the multiverse which causes the two universes to collide. Naruto and his comrades are confused by the new power levels they see in the DBZ characters and vice versa. The Z - fighters are intrigued by the ninjutsu and different types of chakra. They decide to work together when they discover that an ancient evil, which was sealed away in both universes long ago, has been released due to the rift. The combined efforts of Naruto's Rasengan and Goku's Kamehameha could be a crucial element in their fight against this ultimate evil.
One idea could be that Ranma somehow gets transported to the DBZ universe. He could end up training with Goku and learn the Kamehameha wave, but with his own Ranma - style twist. Maybe he uses his martial arts skills from his world to create a new fighting style combined with ki. Another plot could involve the Dragon Balls. Ranma might accidentally make a wish on them that gets him involved in a big battle against a powerful villain in the DBZ world.
One idea could be that Broly ends up in an alternate universe where he has to team up with Goku and the Z - fighters to fight a new, even more powerful enemy. This would force Broly to control his anger and work with his former foes.
For a 'dbz gohan crossover fanfiction', a plot where Gohan crosses into the DC Comics universe would be exciting. He could team up with Superman and learn about different types of powers. Maybe he'd help Superman understand the concept of ki. Another idea is a crossover with the Final Fantasy series. Gohan could find himself in a world full of summons and magic, and use his martial arts to fight alongside the Final Fantasy characters.
A plot could involve a powerful villain who uses the concept of the harem to manipulate Naruto and Goku. The villain might create illusions or put the female characters in danger, forcing our heroes to work together. They would need to not only defeat the villain but also figure out their feelings for the harem members in the process. For example, Naruto's determination and Goku's strength could be combined in unique ways to face these challenges.
Well, a cool plot might be that the Dragon Balls are stolen by an evil force from the Sailor Moon universe. So, the Z Fighters have to travel to that universe to retrieve them. Along the way, they meet the Sailor Scouts and realize they need to work together. The Sailor Scouts could also learn about the power of the Dragon Balls and how they can be used for good. They might even help the Z Fighters in their search by using their unique powers to track down the stolen balls.
Perhaps a villain from DBZ, like Frieza, finds a way to travel to the One Piece universe. He starts enslaving the pirates and taking over islands. Luffy and his crew would be the first to resist, but they quickly realize they need help. Goku and Vegeta then arrive and have to teach the pirates about ki and how to fight against Frieza's powerful minions. It would be a great opportunity to see the different fighting styles blend together.
A cool plot could be a race between The Flash and Gotenks. They both pride themselves on their speed, so it would be interesting to see how that pans out. Also, perhaps Vegeta could befriend Batman as they both have that brooding, serious personality. They could exchange combat and strategy tips. And what if the Dragon Radar was used by the Justice League to track down an energy source that was causing chaos in both universes?