The setting is a common element too. When Ranma crosses over into a different world, the new setting affects his actions and the overall story. For example, if he crosses over into a post - apocalyptic world, he has to use his martial arts in new ways to survive. And of course, the humor that is characteristic of Ranma 1/2 often gets transferred and combined with the humor (if any) of the crossed - over universe.
A third idea is that there's a mix - up with the curses in both worlds. Ranma's curse of changing into a girl when splashed with cold water gets combined with some of the magic in Highschool DxD. This creates a whole new set of problems and adventures for him as he tries to figure out how to deal with this strange new situation while also trying to keep his identity a secret from the new characters he meets.
One common theme is the clash of different worlds. Inuyasha is set in a feudal era full of demons and magic, while Ranma has a more modern (but still very Japanese - culture - influenced) world with martial arts and strange curses. So when the two are crossed over, the difference in their worlds often leads to interesting plotlines. Another theme is the interaction between the characters. For example, Inuyasha's brash and impulsive nature vs Ranma's own quick - tempered but also comical personality can create a lot of humorous and tense situations.
Another issue is the power scaling. If you cross Ranma 1/2 with a series like One Piece where the characters have superhuman abilities, you need to figure out how Ranma's martial arts stack up. You don't want to make him too overpowered or too weak compared to the new characters he's interacting with.
A common theme is the exploration of different power systems. Like how the martial arts in Ranma 1/2 compare to the ki - based powers in Dragon Ball Z.
There's a Ranma 1/2 crossover fanfiction that involves crossing over with the Dragon Ball universe. Ranma's unique fighting style and his ability to change forms due to the curse blend well with the super - powered battles in Dragon Ball. Also, a crossover with Naruto can be really good. Ranma's training and skills can be compared and contrasted with the ninja techniques in Naruto.
One interesting aspect is the character interactions. Ranma's chaotic and comical nature clashes and also meshes in interesting ways with the more reserved or otherworldly characters of Tenchi Muyo!. For example, when Ranma meets Ryoko, their strong personalities can lead to both humorous and tense situations.
Character development. You want to see how the characters from both 'Worm' and 'Ranma' change and grow in the new crossover setting. For example, how Ranma might adapt to the superpower - filled world of 'Worm' or how the 'Worm' characters react to Ranma's martial arts and unique personality.
Well, it could be about Ranma getting transferred to the Highschool DxD world. Maybe he'd get involved in the supernatural battles there, using his martial arts skills. He might team up with Issei and the others, and his unique curse could add an interesting twist to the story. For example, his transformation between male and female forms could lead to some comical or strategic situations during fights.
One great Ranma crossover fanfiction is 'Ranma in the Harry Potter World'. It combines the chaotic world of Ranma with the magic of Harry Potter in an interesting way. The characters from Ranma adapt to the Hogwarts setting, and there are lots of humorous situations as they interact with the wizards and witches.