One common trope is the 'fish - out - of - water' scenario. For example, when Ranma ends up in Yoki's world and has to adapt to new rules and cultures. Another is the 'team - up - against - a - common - enemy' trope. It's always exciting to see how their different abilities come together to defeat a powerful foe. And of course, the 'romantic - subplot' where there might be some sort of love interest developing between characters from the two different franchises.
The 'culture - clash' trope is really popular. Ranma comes from his own unique world in Ranma 1/2, and when he interacts with Yoki's world, there are all sorts of misunderstandings and funny moments. Also, the 'power - exchange' trope can be interesting. Maybe Ranma gives Yoki some martial arts knowledge in exchange for one of Yoki's special powers for a short time, leading to new and unexpected adventures.
There are quite a few out there. Some focus on the comical aspects of a Ranma 1/2 and Yoki crossover. For example, the fanfic where Ranma accidentally gets involved in Yoki's family drama and his attempts to get out of it using his martial arts skills and shape - changing abilities. Others might explore a more serious plot, like when they team up to face a common enemy that threatens both their worlds.
One popular pairing could be Naruto and Akane. Since Naruto has a big heart and is always eager to befriend others, he might be able to break through Akane's tough exterior and they could form an interesting relationship.
One popular trope is the 'fish out of water' trope. When Vash crosses over into another universe, he often has to adapt to new rules and cultures, which makes for some really fun and comical situations. For example, if he crosses into a modern - day Earth setting, his old - fashioned ways and his gun - slinging skills seem out of place but also really cool.
One popular trope is the 'fish out of water' situation. For example, when Soundwave is crossed over into a different universe and has to adapt to new rules and characters. Another is the 'unlikely alliance' trope, where Soundwave teams up with characters he would never usually interact with in the original Tfp setting.
One popular trope is the 'fish - out - of - water' story. Merlin, coming from a medieval world, is suddenly in the modern world of the Avengers. It's funny to see his reactions to things like technology. Another is the 'power - sharing' trope. Merlin's magic and the Avengers' powers combine in unique ways to defeat enemies.
The 'enemies - to - lovers' trope is also common in Draco Malfoy crossover fanfiction. When he crosses over and meets a character from another world, they start off hating each other but gradually fall in love. This can be really exciting to read, especially when they have to overcome their differences in values and backgrounds.
One popular trope is the 'fish out of water' situation. For example, characters from Inuyasha's feudal Japan world find themselves in the modern - day setting of Twilight. Another is the power struggle trope. Inuyasha's demon powers and the vampires' powers in Twilight often lead to interesting power dynamics in the fanfics. There's also the love triangle trope, with the main characters from both universes getting involved in complex relationships.
A common trope is the team - up against a powerful villain. This could be a super - strong demon from Fairy Tail's lore combined with an evil force from Dragon Ball Z. The heroes from both series then have to put aside their differences and work together. Also, the 'rivalry' trope is often used. For instance, Natsu and Goku might start off as rivals, competing to see who is stronger, but then they realize they need to work together. And of course, there's the 'training' trope, where characters from one series train the characters from the other series in their respective fighting styles.
A less common but still interesting trope is the 'reversal of roles'. In some fanfictions, Issei might find himself in a world where his devil powers are seen as something different or even feared in a new way. Maybe in a world of pure angels, he is the outcast and has to prove himself not to be an evil entity. This trope allows for a lot of character exploration and development for Issei.
One popular trope is the characters from both shows teaming up to fight a common supernatural threat. For example, the Winchester brothers from Supernatural joining forces with Rick and his group from The Walking Dead to battle a horde of zombie - like demons.