They could also be used as a running joke in the fanon. I never hear of Doctor Who until I randomly came across the 10th doctor on bbc. Yet a bunch of fanfics in the 90’s early 2000’s would insinuate that Ryoga was a companion. They weren’t crossovers there would just be a random sentence in the fanfic about the characters thinking this Doctor Ryoga kept talking about was an imaginary friend. I figured it was just something a writer made up that others liked and went with.
Isekai’s could be the new running gag instead of this mysterious doctor the charecters could brush off Ryoga’s accounts of mystical adventures he had.
Also it would be interesting to see the characters after the adventure is over. I went into it a bit with my overly cautious hero crossover pitch. However there are also series like Hagure Yūsha no Estetica which not only leads to an entirely new adventure, but also more creative experiments with this concept. What if Ranma is sent to another world, but doesn’t become a hero or gain any of the abilities that come with it.