Perhaps in this fan fiction, the Angels from Evangelion start to invade the Dragon Ball Z universe. The Saiyans, initially thinking they can easily handle the situation, find out that the Angels have some unique abilities that make them very difficult opponents. The Evangelion pilots then come to the Dragon Ball Z universe to help out, and together they have to figure out how to combine their powers and strategies to defeat the Angels.
Well, a cool plot might involve Goku and Shinji Ikari swapping places for a while. Goku, with his positive attitude, could have a big impact on the Evangelion world, while Shinji in the Dragon Ball Z world would be forced to learn about ki and fighting in a very different way than he's used to. This could lead to some really interesting character development for both.
Goku and Asuka would be really interesting. Goku's simple - minded but powerful nature would contrast with Asuka's complex and somewhat arrogant personality. They could have some great banter and also learn from each other in terms of fighting spirit.
One possible interesting plot could be that Super 17 gains new powers that are not in the original series. Maybe he discovers a hidden energy within himself that allows him to time - travel. This could lead to him trying to change key events in the Dragon Ball Z universe, creating all kinds of chaos and new storylines for the other characters to deal with.
An interesting plot could be that the tournament is secretly organized by an evil force. They have placed a curse on the fighters so that if they lose, they lose their powers forever. Goku and his friends don't realize this at first. As the tournament progresses, they start to notice something is wrong. They have to not only win their battles but also figure out how to break the curse before it's too late.
Maybe Naruto, with his godlike powers, enters the Dragon Ball Z universe to stop an evil that neither universe can handle alone.
Well, one interesting concept could be the exploration of alternate timelines where the villains win. For example, a story might show what would happen if Frieza had successfully destroyed Earth. Another concept could be the darkening of the characters' personalities. Maybe Goku turns evil due to some dark magic or mind - control, which would be a huge twist on his usually heroic nature. Also, the fan fiction might delve into the idea of forgotten or suppressed powers that the Z - fighters possess, which are only awakened in these dark and desperate scenarios.
Maybe there's a rift in the multiverse that merges parts of the Dragon Ball Z world and the DC Universe. The Z Fighters and the Justice League initially think they are enemies due to misunderstandings. For instance, when Goku first meets Batman, Batman might mistake Goku's carefree attitude for recklessness. But as they gradually figure out the situation, they team up to close the rift and fight off threats that are emerging from the merged worlds.
A cool plot could be a tournament - style crossover. A powerful being from another dimension decides to host a tournament where fighters from Dragon Ball Z and Devilman are invited. Characters like Vegeta and Devilman's friend Akira Fudo would compete. There could be unexpected alliances and rivalries. For example, Piccolo might initially see Devilman as a threat but later realize they have a common goal of protecting their respective worlds. And in the end, they all have to team up against the true evil that the tournament organizer has in store.
One possible plot could be that Thanos somehow gets his hands on the Dragon Balls. He wishes for the power to snap away half of all life in the universe even more easily. The Z - fighters sense this great imbalance and travel to the Marvel universe to stop him. Goku, being the most powerful Z - fighter, tries to fight Thanos but quickly realizes his strength is different from what he's used to. Vegeta might have to team up with Iron Man to come up with a technological way to counter Thanos' Infinity Stones while Goku trains to reach a new level of Super Saiyan to take on Thanos directly.
A plot might involve the Dragon Balls being scattered across the Bleach universe. Ichigo hears rumors about these powerful orbs and starts looking for them. Meanwhile, Goku and his friends also end up in the Bleach world while searching for the Dragon Balls. They meet the Soul Reapers and at first there's some tension. But as they learn more about each other's goals, they work together. The Soul Reapers are fascinated by Goku's power and training methods, and Goku is intrigued by the concept of souls and the afterlife in the Bleach world.
One interesting fan theory is about the potential hidden powers of some minor characters. For example, some think that Yajirobe might have a secret super form that was never revealed in the original series. It's just a fun idea that fans like to explore in fanfiction.