Well, perhaps there's a powerful enemy that is a threat to both the Ranma and DBZ universes. Ranma and Gohan could team up. Gohan could teach Ranma about Saiyan power and how to access it. Ranma, in turn, could show Gohan some of his unique martial arts techniques. They'd have to combine their skills to defeat this new foe that has the power to destroy both their worlds.
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.
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 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.
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.
One idea could be that Ranma somehow gets involved in the fight against the Dark Kingdom in the Sailor Moon universe. Maybe his curse could be related to the magic there. For example, his female form could be mistaken for a new Sailor Scout by the bad guys.
One idea could be that the Ranma characters get transported to the Bubblegum Crisis world. Ranma could be confused by the high - tech setting and the existence of Boomers. Maybe he would initially try to use his martial arts against them, thinking they are just some strange opponents. Another plot could involve the Knight Sabers somehow getting involved in a martial arts tournament in the Ranma world. They would have to adapt to the different fighting styles and rules there.
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?
One idea could be a battle between Goku and Superman to determine who is the strongest in the universe. They might start off as rivals but then realize they have more in common than they thought and team up against a greater threat like Darkseid and Frieza combined.
One idea could be that Goku ends up in the world of Aladdin. He might mistake the flying carpet for a new kind of Ki - powered vehicle. And he could use his martial arts skills to help Aladdin fight off Jafar's minions in a really action - packed way.
Another interesting concept is to cross them over with a fantasy world. Saiyans could be in a world full of magic and mythical creatures. Picture Vegeta having to deal with dragons that can talk and cast spells. Their Saiyan pride might get in the way at first when they realize that their brute strength isn't always enough in this new world.